diff --git a/include/pocketpy/codeobject.h b/include/pocketpy/codeobject.h
index 2d2aa468..04495343 100644
--- a/include/pocketpy/codeobject.h
+++ b/include/pocketpy/codeobject.h
@@ -76,6 +76,7 @@ struct CodeObject {
     
     small_vector_2<PyVar, 8> consts;         // constants
     small_vector_2<StrName, 8> varnames;     // local variables
+    int nlocals;                             // varnames.size()
 
     NameDictInt varnames_inv;
     std::vector<CodeBlock> blocks;
diff --git a/include/pocketpy/frame.h b/include/pocketpy/frame.h
index cdf7c1f2..1f76e5e4 100644
--- a/include/pocketpy/frame.h
+++ b/include/pocketpy/frame.h
@@ -14,7 +14,7 @@ struct FastLocals{
     const CodeObject* co;
     PyVar* a;
 
-    int size() const{ return co->varnames.size();}
+    int size() const{ return co->nlocals;}
 
     PyVar& operator[](int i){ return a[i]; }
     PyVar operator[](int i) const { return a[i]; }
diff --git a/src/codeobject.cpp b/src/codeobject.cpp
index b45cc3bd..76841756 100644
--- a/src/codeobject.cpp
+++ b/src/codeobject.cpp
@@ -3,7 +3,7 @@
 namespace pkpy{
 
     CodeObject::CodeObject(std::shared_ptr<SourceData> src, const Str& name):
-        src(src), name(name), start_line(-1), end_line(-1) {
+        src(src), name(name), nlocals(0), start_line(-1), end_line(-1) {
             blocks.push_back(CodeBlock(CodeBlockType::NO_BLOCK, -1, 0, 0));
         }
 
diff --git a/src/compiler.cpp b/src/compiler.cpp
index 5fade046..cc8aaf66 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -41,7 +41,7 @@ namespace pkpy{
 
         // some check here
         auto& codes = ctx()->co->codes;
-        if(ctx()->co->varnames.size() > PK_MAX_CO_VARNAMES){
+        if(ctx()->co->nlocals > PK_MAX_CO_VARNAMES){
             SyntaxError("maximum number of local variables exceeded");
         }
         if(ctx()->co->consts.size() > 65530){
diff --git a/src/expr.cpp b/src/expr.cpp
index 4d5516a5..c01d3496 100644
--- a/src/expr.cpp
+++ b/src/expr.cpp
@@ -111,6 +111,7 @@ namespace pkpy{
         int index = co->varnames_inv.try_get(name);
         if(index >= 0) return index;
         co->varnames.push_back(name);
+        co->nlocals++;
         index = co->varnames.size() - 1;
         co->varnames_inv.set(name, index);
         return index;
diff --git a/src/vm.cpp b/src/vm.cpp
index c984c0f9..1308c256 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -949,7 +949,6 @@ void VM::__unpack_as_dict(ArgsView args, Dict& dict){
 
 void VM::__prepare_py_call(PyVar* buffer, ArgsView args, ArgsView kwargs, const FuncDecl_& decl){
     const CodeObject* co = decl->code.get();
-    int co_nlocals = co->varnames.size();
     int decl_argc = decl->args.size();
 
     if(args.size() < decl_argc){
@@ -960,7 +959,7 @@ void VM::__prepare_py_call(PyVar* buffer, ArgsView args, ArgsView kwargs, const
 
     int i = 0;
     // prepare args
-    memset(buffer, 0, co_nlocals * sizeof(PyVar));
+    memset(buffer, 0, co->nlocals * sizeof(PyVar));
     for(int index: decl->args) buffer[index] = args[i++];
     // prepare kwdefaults
     for(auto& kv: decl->kwargs) buffer[kv.index] = kv.value;
@@ -1036,22 +1035,20 @@ PyVar VM::vectorcall(int ARGC, int KWARGC, bool op_call){
 
         const Function& fn = PK_OBJ_GET(Function, callable);
         const CodeObject* co = fn.decl->code.get();
-        int co_nlocals = co->varnames.size();
 
         switch(fn.decl->type){
-            case FuncType::UNSET: PK_FATAL_ERROR(); break;
             case FuncType::NORMAL:
                 __prepare_py_call(__vectorcall_buffer, args, kwargs, fn.decl);
                 // copy buffer back to stack
-                s_data.reset(_base + co_nlocals);
-                for(int j=0; j<co_nlocals; j++) _base[j] = __vectorcall_buffer[j];
+                s_data.reset(_base + co->nlocals);
+                for(int j=0; j<co->nlocals; j++) _base[j] = __vectorcall_buffer[j];
                 break;
             case FuncType::SIMPLE:
                 if(args.size() != fn.decl->args.size()) TypeError(_S(co->name, "() takes ", fn.decl->args.size(), " positional arguments but ", args.size(), " were given"));
                 if(!kwargs.empty()) TypeError(_S(co->name, "() takes no keyword arguments"));
                 // [callable, <self>, args..., local_vars...]
                 //      ^p0                    ^p1      ^_sp
-                s_data.reset(_base + co_nlocals);
+                s_data.reset(_base + co->nlocals);
                 // initialize local variables to PY_NULL
                 memset(p1, 0, (char*)s_data._sp - (char*)p1);
                 break;
@@ -1065,8 +1062,13 @@ PyVar VM::vectorcall(int ARGC, int KWARGC, bool op_call){
                 s_data.reset(p0);
                 return __py_generator(
                     Frame(nullptr, co, fn._module, callable, nullptr),
-                    ArgsView(__vectorcall_buffer, __vectorcall_buffer + co_nlocals)
+                    ArgsView(__vectorcall_buffer, __vectorcall_buffer + co->nlocals)
                 );
+#if PK_DEBUG_EXTRA_CHECK
+            default: PK_FATAL_ERROR(); break;
+#else
+            default: PK_UNREACHABLE()
+#endif
         };
 
         // simple or normal
@@ -1080,7 +1082,7 @@ PyVar VM::vectorcall(int ARGC, int KWARGC, bool op_call){
         const auto& f = PK_OBJ_GET(NativeFunc, callable);
         PyVar ret;
         if(f.decl != nullptr){
-            int co_nlocals = f.decl->code->varnames.size();
+            int co_nlocals = f.decl->code->nlocals;
             __prepare_py_call(__vectorcall_buffer, args, kwargs, f.decl);
             // copy buffer back to stack
             s_data.reset(_base + co_nlocals);