diff --git a/include/vm/internals/reader.h b/include/vm/internals/reader.h
index 8764cd7..dbfd276 100644
--- a/include/vm/internals/reader.h
+++ b/include/vm/internals/reader.h
@@ -7,6 +7,8 @@
 #include <stdio.h>
 #include <vm/fzl_vm_reader.h>
 
+extern char internal_error[256];
+
 bool vm_rd_char(FILE *fd, size_t count, char *outbuf);
 bool vm_rd_u16(FILE *fd, size_t count, uint16_t *outbuf);
 bool vm_rd_word(FILE *fd, size_t count, size_t *outbuf);
diff --git a/src/compiler/fzl_writer.c b/src/compiler/fzl_writer.c
index 38e4e48..c9994d0 100644
--- a/src/compiler/fzl_writer.c
+++ b/src/compiler/fzl_writer.c
@@ -2,7 +2,6 @@
 #include <compiler/fzl_writer.h>
 #include <compiler/internals/writer.h>
 #include <compiler/utils/fzl_const_pool.h>
-#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -14,11 +13,11 @@ char wrlast_err[256];
 fzl_writer_t *fzl_mkwriter(const char *filepath)
 {
   if (strlen(filepath) < 1)
-    return NULL;
+    return nullptr;
 
   fzl_writer_t *writer = malloc(sizeof(fzl_writer_t));
   if (!writer)
-    return NULL;
+    return nullptr;
 
   writer->filepath = filepath;
 
@@ -38,7 +37,7 @@ int fzl_write(fzl_writer_t *writer, fzl_gen_t *gen)
     return -1;
   }
 
-  const char hdr[5] = {'F', 'Z', 'L', 0x0, 0x2};
+  constexpr char hdr[5] = {'F', 'Z', 'L', 0x0, 0x2};
   fwrite(hdr, sizeof(char), 5, fd);
   fwrite(&gen->consts->size, sizeof(uint16_t), 1, fd);
   fwrite(&gen->extern_libs_count, sizeof(uint16_t), 1, fd);
diff --git a/src/vm/fzl_vm.c b/src/vm/fzl_vm.c
index bb46ab3..b3d2d0c 100644
--- a/src/vm/fzl_vm.c
+++ b/src/vm/fzl_vm.c
@@ -1,11 +1,8 @@
-#include <common/fzl_builtins.h>
 #include <common/fzl_opcodes.h>
 #include <common/fzl_types.h>
 #include <common/fzl_value.h>
 #include <compiler/utils/fzl_const_pool.h>
-#include <stdbool.h>
 #include <stddef.h>
-#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -16,11 +13,10 @@
 #include <vm/internals/opcode.h>
 
 char last_err[256];
+char internal_error[256];
 size_t last_jmp_ip       = 0;
 size_t total_jmp_on_same = 0;
 
-static char *expect_hdr = "FZL\0\2";
-
 fzl_vm_t *fzl_mkvm(fzl_vm_reader_t *reader)
 {
   if (!reader)
diff --git a/src/vm/fzl_vm_reader.c b/src/vm/fzl_vm_reader.c
index 09bbdbe..037c467 100644
--- a/src/vm/fzl_vm_reader.c
+++ b/src/vm/fzl_vm_reader.c
@@ -35,8 +35,12 @@ fzl_vm_reader_t *fzl_vm_mkreader(char *filepath)
 
 void wr_validation_err(char *section)
 {
-  snprintf(reader_err, sizeof(reader_err),
-           "init error - section '%s' is not valid", section);
+  if (internal_error[0])
+    snprintf(reader_err, sizeof(reader_err), "init error - section '%s': %s",
+             section, internal_error);
+  else
+    snprintf(reader_err, sizeof(reader_err),
+             "init error - section '%s' is not valid", section);
 }
 
 bool fzl_vm_read(fzl_vm_reader_t *reader)
@@ -137,7 +141,26 @@ void fzl_vm_freereader(fzl_vm_reader_t *reader)
     free(reader->extern_libs);
 
   if (reader->extern_types)
+  {
+    for (uint16_t i = 0; i < reader->extern_types_count; i++)
+    {
+      free(reader->extern_types[i].name);
+
+      if (reader->extern_types[i].kind == FZL_EXTERN_KIND_STRUCT)
+      {
+        fzl_extern_struct_field_t *fields =
+            reader->extern_types[i].structure.fields;
+        for (uint16_t j = 0; j < reader->extern_types[i].structure.fields_count;
+             j++)
+        {
+          if (fields[j].name)
+            free(fields[j].name);
+        }
+      }
+    }
+
     free(reader->extern_types);
+  }
 
   if (reader->extern_fns)
     free(reader->extern_fns);
diff --git a/src/vm/main.c b/src/vm/main.c
index 8f91556..2b55e0a 100644
--- a/src/vm/main.c
+++ b/src/vm/main.c
@@ -161,6 +161,7 @@ int main(int argc, char **argv)
   }
 
   free(args);
+  fzl_vm_freereader(reader);
   fzl_freevm(vm);
 
   return 0;
diff --git a/src/vm/reader/utility.c b/src/vm/reader/utility.c
index 0054bac..3f16d50 100644
--- a/src/vm/reader/utility.c
+++ b/src/vm/reader/utility.c
@@ -10,7 +10,7 @@ bool vm_rd_char(FILE *fd, size_t count, char *o)
   if (!fd || !o)
     return false;
 
-  return fread(o, sizeof(char), count, fd) == (sizeof(char) * count);
+  return fread(o, sizeof(char), count, fd) == count;
 }
 
 // Read 16-byte uints with red bytes check
@@ -19,7 +19,7 @@ bool vm_rd_u16(FILE *fd, size_t count, uint16_t *o)
   if (!fd || !o)
     return false;
 
-  return fread(o, sizeof(uint16_t), count, fd) == (sizeof(uint16_t) * count);
+  return fread(o, sizeof(uint16_t), count, fd) == count;
 }
 
 // Read machine word-size uints with red bytes check
@@ -28,5 +28,5 @@ bool vm_rd_word(FILE *fd, size_t count, size_t *o)
   if (!fd || !o)
     return false;
 
-  return fread(o, sizeof(size_t), count, fd) == (sizeof(size_t) * count);
+  return fread(o, sizeof(size_t), count, fd) == count;
 }
diff --git a/src/vm/reader/vm_rd_consts.c b/src/vm/reader/vm_rd_consts.c
index 593f7e5..4245d7b 100644
--- a/src/vm/reader/vm_rd_consts.c
+++ b/src/vm/reader/vm_rd_consts.c
@@ -3,6 +3,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <vm/internals/reader.h>
@@ -19,7 +20,10 @@ bool vm_rd_consts(fzl_vm_reader_t *reader, FILE *fd)
 
   reader->consts = malloc(sizeof(fzl_value_t) * consts_count);
   if (!reader->consts)
+  {
+    snprintf(internal_error, sizeof(internal_error), "out of memory");
     return false;
+  }
 
   memset(reader->consts, 0, sizeof(fzl_value_t) * consts_count);
 
@@ -31,19 +35,32 @@ bool vm_rd_consts(fzl_vm_reader_t *reader, FILE *fd)
     memset(contents, 0, sizeof(contents));
 
     ok = vm_rd_u16(fd, 1, &const_size);
-    if (!ok || const_size < 1)
+    if (!ok)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read size of const #%u", i);
       return false;
+    }
 
     ok = vm_rd_char(fd, const_size, contents);
     if (!ok)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read const #%u", i);
       return false;
+    }
 
-    char *val = malloc(sizeof(char) * const_size);
+    char *val = malloc(sizeof(char) * const_size + 1);
     if (!val)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "out of memory on const #%u allocation", i);
       return false;
+    }
 
-    memset(val, 0, sizeof(char) * const_size);
+    memset(val, 0, sizeof(char) * const_size + 1);
     memcpy(val, contents, sizeof(char) * const_size);
+
     reader->consts[i] = (fzl_value_t){
         .type = FZL_TYPE_STRING, .as.s.value = val, .as.s.length = const_size};
   }
diff --git a/src/vm/reader/vm_rd_extern_libs.c b/src/vm/reader/vm_rd_extern_libs.c
index d924c39..a6698db 100644
--- a/src/vm/reader/vm_rd_extern_libs.c
+++ b/src/vm/reader/vm_rd_extern_libs.c
@@ -1,6 +1,7 @@
 #include <common/fzl_extern.h>
 #include <stdbool.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <vm/internals/reader.h>
@@ -17,7 +18,10 @@ bool vm_rd_extern_libs(fzl_vm_reader_t *reader, FILE *fd)
 
   reader->extern_libs = malloc(sizeof(char *) * libs_count);
   if (!reader->extern_libs)
+  {
+    snprintf(internal_error, sizeof(internal_error), "out of memory");
     return false;
+  }
 
   memset(reader->extern_libs, 0, sizeof(char *) * libs_count);
 
@@ -30,17 +34,29 @@ bool vm_rd_extern_libs(fzl_vm_reader_t *reader, FILE *fd)
 
     ok = vm_rd_u16(fd, 1, &name_sz);
     if (!ok || name_sz < 1)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read lib #%u name size", i);
       return false;
+    }
 
     ok = vm_rd_char(fd, name_sz, name);
     if (!ok)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read lib #%u name", i);
       return false;
+    }
 
-    char *val = malloc(sizeof(char) * name_sz);
+    char *val = malloc(sizeof(char) * name_sz + 1);
     if (!val)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "out of memory on lib #%u name allocation", i);
       return false;
+    }
 
-    memset(val, 0, sizeof(char) * name_sz);
+    memset(val, 0, sizeof(char) * name_sz + 1);
     memcpy(val, name, sizeof(char) * name_sz);
     reader->extern_libs[i] = val;
   }
diff --git a/src/vm/reader/vm_rd_extern_types.c b/src/vm/reader/vm_rd_extern_types.c
index ca644a7..f1a1728 100644
--- a/src/vm/reader/vm_rd_extern_types.c
+++ b/src/vm/reader/vm_rd_extern_types.c
@@ -20,7 +20,10 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
 
   reader->extern_types = malloc(sizeof(fzl_extern_type_t) * types_count);
   if (!reader->extern_types)
+  {
+    snprintf(internal_error, sizeof(internal_error), "out of memory");
     return false;
+  }
 
   memset(reader->extern_types, 0, sizeof(fzl_extern_type_t) * types_count);
 
@@ -31,27 +34,50 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
 
     uint16_t lib_slot;
     if (!vm_rd_u16(fd, 1, &lib_slot))
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read lib slot of type #%u", i);
       return false;
+    }
+
     type->lib_slot = lib_slot;
 
     char kind;
     if (!vm_rd_char(fd, 1, &kind))
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read type #%u kind", i);
       return false;
+    }
+
     type->kind = (fzl_extern_kind_t)kind;
 
     uint16_t name_sz;
     if (!vm_rd_u16(fd, 1, &name_sz) || name_sz < 1)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read type #%u name size", i);
       return false;
+    }
 
     if (!vm_rd_char(fd, name_sz, namebuf))
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read type #%u name", i);
       return false;
+    }
 
-    char *val = malloc(sizeof(char) * name_sz);
+    char *val = malloc(sizeof(char) * name_sz + 1);
     if (!val)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "out of memory on type #%u name allocation", i);
       return false;
+    }
+
     type->name = val;
 
-    memset(val, 0, sizeof(char) * name_sz);
+    memset(val, 0, sizeof(char) * name_sz + 1);
     memcpy(val, namebuf, sizeof(char) * name_sz);
 
     if (type->kind == FZL_EXTERN_KIND_HANDLE)
@@ -66,21 +92,36 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
 
       char is_primitive;
       if (!vm_rd_char(fd, 1, &is_primitive))
+      {
+        snprintf(internal_error, sizeof(internal_error),
+                 "failed to read type #%u primitivity flag", i);
         return false;
+      }
+
       typeref->is_primitive = (bool)is_primitive;
 
       if ((bool)is_primitive)
       {
         char primitive;
         if (!vm_rd_char(fd, 1, &primitive))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u primitive", i);
           return false;
+        }
+
         typeref->primitive = (fzl_type_t)primitive;
       }
       else
       {
         uint16_t slot;
         if (!vm_rd_u16(fd, 1, &slot))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u slot index", i);
           return false;
+        }
+
         typeref->type_slot = slot;
       }
 
@@ -88,16 +129,28 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
     }
 
     if (type->kind != FZL_EXTERN_KIND_STRUCT)
+    {
+      snprintf(internal_error, sizeof(internal_error), "unknown type #%u kind",
+               i);
       return false;
+    }
 
     uint16_t fields_count;
     if (!vm_rd_u16(fd, 1, &fields_count))
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read type #%u structure fields count", i);
       return false;
+    }
 
     fzl_extern_struct_field_t *fields =
         malloc(sizeof(fzl_extern_struct_field_t) * fields_count);
     if (!fields)
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "out of memory on type #%u structure fields allocation", i);
       return false;
+    }
 
     memset(fields, 0, sizeof(fzl_extern_struct_field_t) * fields_count);
     type->structure.fields       = fields;
@@ -109,22 +162,38 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
 
       char has_name;
       if (!vm_rd_char(fd, 1, &has_name))
+      {
+        snprintf(internal_error, sizeof(internal_error),
+                 "failed to read type #%u field #%u name exists flag", i, j);
         return false;
+      }
 
       if (has_name)
       {
         memset(namebuf, 0, sizeof(namebuf));
         if (!vm_rd_u16(fd, 1, &name_sz))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u field #%u name size", i, j);
           return false;
+        }
 
         if (!vm_rd_char(fd, name_sz, namebuf))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u field #%u name", i, j);
           return false;
+        }
 
-        char *fval = malloc(sizeof(char) * name_sz);
+        char *fval = malloc(sizeof(char) * name_sz + 1);
         if (!fval)
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "out of memory on type #%u field #%u name allocation", i, j);
           return false;
+        }
 
-        memset(fval, 0, sizeof(char) * name_sz);
+        memset(fval, 0, sizeof(char) * name_sz + 1);
         memcpy(fval, namebuf, sizeof(char) * name_sz);
 
         f->name = fval;
@@ -132,21 +201,36 @@ bool vm_rd_extern_types(fzl_vm_reader_t *reader, FILE *fd)
 
       char is_primitive;
       if (!vm_rd_char(fd, 1, &is_primitive))
+      {
+        snprintf(internal_error, sizeof(internal_error),
+                 "failed to read type #%u flag #%u primitivity flag", i, j);
         return false;
+      }
+
       f->type.is_primitive = (bool)is_primitive;
 
       if (f->type.is_primitive)
       {
         char ftype;
         if (!vm_rd_char(fd, 1, &ftype))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u field #%u primitive", i, j);
           return false;
+        }
+
         f->type.primitive = (fzl_type_t)ftype;
       }
       else
       {
         uint16_t fslot;
         if (!vm_rd_u16(fd, 1, &fslot))
+        {
+          snprintf(internal_error, sizeof(internal_error),
+                   "failed to read type #%u field #%u slot index", i, j);
           return false;
+        }
+
         f->type.type_slot = fslot;
       }
     }
diff --git a/src/vm/reader/vm_rd_header.c b/src/vm/reader/vm_rd_header.c
index bf14fe7..6f226a0 100644
--- a/src/vm/reader/vm_rd_header.c
+++ b/src/vm/reader/vm_rd_header.c
@@ -13,8 +13,7 @@ bool vm_rd_header(fzl_vm_reader_t *reader, FILE *fd)
     return false;
 
   char chkbuf[5];
-  bool ok = vm_rd_char(fd, 5, chkbuf);
-  if (!ok)
+  if (!vm_rd_char(fd, 5, chkbuf))
     return false;
 
   if (memcmp(chkbuf, hdr, hdr_size) != 0)
diff --git a/src/vm/reader/vm_rd_ops.c b/src/vm/reader/vm_rd_ops.c
index 9e5938c..30126bb 100644
--- a/src/vm/reader/vm_rd_ops.c
+++ b/src/vm/reader/vm_rd_ops.c
@@ -15,13 +15,20 @@ bool vm_rd_ops(fzl_vm_reader_t *reader, FILE *fd)
 
   reader->ops = malloc(sizeof(size_t) * ops_count);
   if (!reader->ops)
+  {
+    snprintf(internal_error, sizeof(internal_error), "out of memory");
     return false;
+  }
 
   for (size_t i = 0; i < ops_count; i++)
   {
     size_t opcode;
     if (!vm_rd_word(fd, 1, &opcode))
+    {
+      snprintf(internal_error, sizeof(internal_error),
+               "failed to read opcode #%zu", i);
       return false;
+    }
 
     reader->ops[i] = opcode;
   }
diff --git a/src/vm/reader/vm_rd_sections_sz.c b/src/vm/reader/vm_rd_sections_sz.c
index 5fbe407..e9b57de 100644
--- a/src/vm/reader/vm_rd_sections_sz.c
+++ b/src/vm/reader/vm_rd_sections_sz.c
@@ -1,6 +1,5 @@
-#include <stdbool.h>
-#include <stdint.h>
 #include <vm/internals/reader.h>
+#include <wchar.h>
 
 // Reader sizes section
 bool vm_rd_sections_sz(fzl_vm_reader_t *reader, FILE *fd)
@@ -8,22 +7,40 @@ bool vm_rd_sections_sz(fzl_vm_reader_t *reader, FILE *fd)
   if (!reader || !fd)
     return false;
 
-  bool ok = vm_rd_u16(fd, 1, (uint16_t *)&reader->consts_count);
-  if (!ok)
+  if (!vm_rd_u16(fd, 1, &reader->consts_count))
+  {
+    snprintf(internal_error, sizeof(internal_error),
+             "failed to read constants table size");
     return false;
+  }
 
-  ok = vm_rd_u16(fd, 1, &reader->extern_libs_count);
-  if (!ok)
+  if (!vm_rd_u16(fd, 1, &reader->extern_libs_count))
+  {
+    snprintf(internal_error, sizeof(internal_error),
+             "failed to read external libs count");
     return false;
+  }
 
-  ok = vm_rd_u16(fd, 1, &reader->extern_types_count);
-  if (!ok)
+  if (!vm_rd_u16(fd, 1, &reader->extern_types_count))
+  {
+    snprintf(internal_error, sizeof(internal_error),
+             "failed to read external types count");
     return false;
+  }
 
-  ok = vm_rd_u16(fd, 1, &reader->extern_fns_count);
-  if (!ok)
+  if (!vm_rd_u16(fd, 1, &reader->extern_fns_count))
+  {
+    snprintf(internal_error, sizeof(internal_error),
+             "failed to read external functions count");
     return false;
+  }
 
-  ok = vm_rd_word(fd, 1, &reader->ops_count);
-  return ok;
+  if (!vm_rd_word(fd, 1, &reader->ops_count))
+  {
+    snprintf(internal_error, sizeof(internal_error),
+             "failed to read opcodes count");
+    return false;
+  }
+
+  return true;
 }