diff --git a/amalgamate.py b/amalgamate.py
index 1a72f240..1320077c 100644
--- a/amalgamate.py
+++ b/amalgamate.py
@@ -2,7 +2,7 @@ with open("src/opcodes.h", "rt", encoding='utf-8') as f:
 	OPCODES_TEXT = f.read()
 
 pipeline = [
-	["__stl__.h", "str.h", "builtins.h", "error.h"],
+	["__stl__.h", "str.h", "safestl.h", "builtins.h", "error.h"],
 	["obj.h", "iter.h", "parser.h", "pointer.h", "codeobject.h"],
 	["vm.h", "compiler.h", "repl.h"],
 	["pocketpy.h"]
diff --git a/src/codeobject.h b/src/codeobject.h
index f3353c5d..ea19fbe5 100644
--- a/src/codeobject.h
+++ b/src/codeobject.h
@@ -139,7 +139,7 @@ public:
         return code->src->snapshot(line);
     }
 
-    int stackSize() const {
+    inline int stackSize() const {
         return s_data.size();
     }
 
@@ -148,6 +148,7 @@ public:
     }
 
     inline PyVar __pop(){
+        if(s_data.empty()) throw std::runtime_error("s_data.empty() is true");
         PyVar v = s_data.back();
         s_data.pop_back();
         return v;
@@ -160,10 +161,11 @@ public:
     }
 
     inline PyVar topValue(VM* vm){
+        if(s_data.empty()) throw std::runtime_error("s_data.empty() is true");
         return __deref_pointer(vm, s_data.back());
     }
 
-    inline PyVar topNValue(VM* vm, int n=-1){
+    inline PyVar __topValueN(VM* vm, int n=-1){
         return __deref_pointer(vm, s_data[s_data.size() + n]);
     }
 
diff --git a/src/error.h b/src/error.h
index a81c9019..d6e0f732 100644
--- a/src/error.h
+++ b/src/error.h
@@ -1,7 +1,6 @@
 #pragma once
 
-#include "__stl__.h"
-#include "str.h"
+#include "safestl.h"
 
 class NeedMoreLines {
 public:
diff --git a/src/obj.h b/src/obj.h
index 6a60ac73..ec7c6f2f 100644
--- a/src/obj.h
+++ b/src/obj.h
@@ -1,7 +1,6 @@
 #pragma once
 
-#include "__stl__.h"
-#include "str.h"
+#include "safestl.h"
 
 typedef int64_t _Int;
 typedef double _Float;
@@ -13,43 +12,12 @@ const _Float _FLOAT_INF_NEG = -INFINITY;
 
 #define PK_VERSION "0.2.3"
 
-class PyObject;
 class CodeObject;
 class BasePointer;
 class VM;
-
 class PkExportedResource {};
 
-typedef std::shared_ptr<PyObject> PyVar;
-typedef PyVar PyVarOrNull;
-
-class PyVarList: public std::vector<PyVar> {
-    PyVar& at(size_t) = delete;
-
-    inline void __checkIndex(size_t i) const {
-        if (i >= size()){
-            auto msg = "std::vector index out of range, " + std::to_string(i) + " not in [0, " + std::to_string(size()) + ")";
-            throw std::out_of_range(msg);
-        }
-    }
-public:
-    PyVar& operator[](size_t i) {
-        __checkIndex(i);
-        return std::vector<PyVar>::operator[](i);
-    }
-
-    const PyVar& operator[](size_t i) const {
-        __checkIndex(i);
-        return std::vector<PyVar>::operator[](i);
-    }
-
-    // define constructors the same as std::vector
-    using std::vector<PyVar>::vector;
-};
-
-typedef std::unordered_map<_Str, PyVar> PyVarDict;
 typedef std::shared_ptr<const BasePointer> _Pointer;
-
 typedef PyVar (*_CppFunc)(VM*, PyVarList);
 typedef std::shared_ptr<CodeObject> _Code;
 
diff --git a/src/safestl.h b/src/safestl.h
new file mode 100644
index 00000000..b3fedf09
--- /dev/null
+++ b/src/safestl.h
@@ -0,0 +1,54 @@
+#pragma once
+
+#include "__stl__.h"
+#include "str.h"
+
+class PyObject;
+typedef std::shared_ptr<PyObject> PyVar;
+typedef PyVar PyVarOrNull;
+
+class PyVarList: public std::vector<PyVar> {
+    PyVar& at(size_t) = delete;
+
+    inline void __checkIndex(size_t i) const {
+        if (i >= size()){
+            auto msg = "std::vector index out of range, " + std::to_string(i) + " not in [0, " + std::to_string(size()) + ")";
+            throw std::out_of_range(msg);
+        }
+    }
+public:
+    PyVar& operator[](size_t i) {
+        __checkIndex(i);
+        return std::vector<PyVar>::operator[](i);
+    }
+
+    const PyVar& operator[](size_t i) const {
+        __checkIndex(i);
+        return std::vector<PyVar>::operator[](i);
+    }
+
+    // define constructors the same as std::vector
+    using std::vector<PyVar>::vector;
+};
+
+
+class PyVarDict: public std::unordered_map<_Str, PyVar> {
+    PyVar& at(const _Str&) = delete;
+
+public:
+    PyVar& operator[](const _Str& key) {
+        return std::unordered_map<_Str, PyVar>::operator[](key);
+    }
+
+    const PyVar& operator[](const _Str& key) const {
+        auto it = find(key);
+        if (it == end()){
+            auto msg = "std::unordered_map key not found, '" + key.str() + "'";
+            throw std::out_of_range(msg);
+        }
+        return it->second;
+    }
+
+    // define constructors the same as std::unordered_map
+    using std::unordered_map<_Str, PyVar>::unordered_map;
+};
\ No newline at end of file
diff --git a/src/vm.h b/src/vm.h
index b9f186b2..ebbf7018 100644
--- a/src/vm.h
+++ b/src/vm.h
@@ -124,7 +124,7 @@ private:
             } break;
             case OP_LIST_APPEND: {
                 PyVar obj = frame->popValue(this);
-                PyVar list = frame->topNValue(this, -2);
+                PyVar list = frame->__topValueN(this, -2);
                 fastCall(list, "append", {list, obj});
             } break;
             case OP_STORE_FUNCTION: