diff --git a/src/arena.cpp b/src/arena.cpp
index f3485b3..d7e0889 100644
--- a/src/arena.cpp
+++ b/src/arena.cpp
@@ -56,6 +56,8 @@ Result<GcRootList*> Arena::gc_root(GcRootList* node) {
 }
 
 Result<PodObject*> Arena::gc_pod(PodObject* obj) {
+  if (obj == 0) return 0;
+
   switch (obj->header.tag) {
     case Tag::Nil:
       return gc_nil((PodNil*)obj);
@@ -89,8 +91,8 @@ Result<PodObject*> Arena::gc_pod(PodObject* obj) {
       return gc_stdlib_function((PodStdlibFunction*)obj);
     case Tag::Module:
       return gc_module((PodModule*)obj);
-    case Tag::Stack:
-      return gc_stack((PodStack*)obj);
+    case Tag::StackFrame:
+      return gc_stack((PodStackFrame*)obj);
     case Tag::Error:
       return gc_error((PodError*)obj);
   }
@@ -251,9 +253,10 @@ Result<PodObject*> Arena::gc_module(PodModule* obj) {
   return nobj;
 }
 
-Result<PodObject*> Arena::gc_stack(PodStack* obj) {
-  auto nobj = TRY(alloc<PodStack>(sizeof(OffPtr<PodObject>) * obj->size));
-  nobj->header.tag = Tag::Stack;
+Result<PodObject*> Arena::gc_stack(PodStackFrame* obj) {
+  auto nobj = TRY(alloc<PodStackFrame>(sizeof(OffPtr<PodObject>) * obj->size));
+  nobj->header.tag = Tag::StackFrame;
+  nobj->parent = TRY(gc_pod(obj->parent.get()));
   nobj->size = obj->size;
   nobj->top = obj->top;
   for (uint64_t i = 0; i < obj->top; i++) {
diff --git a/src/arena.hpp b/src/arena.hpp
index 2d8fd8f..40e67d6 100644
--- a/src/arena.hpp
+++ b/src/arena.hpp
@@ -159,7 +159,7 @@ class Arena {
   Result<PodObject*> gc_function(PodFunction* obj);
   Result<PodObject*> gc_stdlib_function(PodStdlibFunction* obj);
   Result<PodObject*> gc_module(PodModule* obj);
-  Result<PodObject*> gc_stack(PodStack* obj);
+  Result<PodObject*> gc_stack(PodStackFrame* obj);
   Result<PodObject*> gc_error(PodError* obj);
 
   void add_root(GcRootList* node) { _gcroot.insert(node); }
diff --git a/src/common.cpp b/src/common.cpp
index 8f67b29..0cc9ba3 100644
--- a/src/common.cpp
+++ b/src/common.cpp
@@ -42,8 +42,8 @@ Result<Value> Value::create(PodObject* obj) {
       return Value(TRY(StdlibFunction::create((PodStdlibFunction*)obj)));
     case Tag::Module:
       return Value(TRY(Module::create((PodModule*)obj)));
-    case Tag::Stack:
-      return Value(TRY(Stack::create((PodStack*)obj)));
+    case Tag::StackFrame:
+      return Value(TRY(StackFrame::create((PodStackFrame*)obj)));
     case Tag::Error:
       return Value(TRY(Error::create((PodError*)obj)));
   };
@@ -302,17 +302,33 @@ Result<short> Module::cmp(const Module& rhs) const {
   return res;
 }
 
-Result<Value> Stack::copy_value() const {
-  return Value(Stack(TRY(_value.copy())));
+Result<StackFrame> StackFrame::create(uint64_t size) {
+  auto nil = Value(TRY(Nil::create()));
+  auto pod = TRY(arena_alloc<PodStackFrame>(sizeof(OffPtr<PodObject>) * size));
+  pod->header.tag = Tag::StackFrame;
+
+  pod->parent = nil.pod();
+  pod->size = size;
+  pod->top = 0;
+
+  for (uint64_t i = 0; i < size; i++) {
+    pod->data[i] = 0;
+  }
+
+  return StackFrame(TRY(MkGcRoot(pod)));
 }
 
-Result<Value> Stack::get(uint64_t idx) const {
+Result<Value> StackFrame::copy_value() const {
+  return Value(StackFrame(TRY(_value.copy())));
+}
+
+Result<Value> StackFrame::get(uint64_t idx) const {
   if (idx >= _value->top) return ERROR(KeyError);
 
   return Value::create(_value->data[idx].get());
 }
 
-Result<void> Stack::set(uint64_t idx, const Value& val) {
+Result<void> StackFrame::set(uint64_t idx, const Value& val) {
   if (idx >= _value->size) return ERROR(KeyError);
 
   if (idx >= _value->top) {
@@ -323,7 +339,7 @@ Result<void> Stack::set(uint64_t idx, const Value& val) {
   return Result<void>();
 }
 
-Result<void> Stack::settop(uint64_t idx) {
+Result<void> StackFrame::settop(uint64_t idx) {
   uint64_t top = _value->top;
   for (uint64_t i = std::min(top, idx); i < std::max(top, idx); i++) {
     _value->data[i] = 0;
diff --git a/src/common.hpp b/src/common.hpp
index de4670c..40c8059 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -32,7 +32,7 @@ class Dict;
 class ByteArray;
 class Writer;
 class Opcode;
-class Stack;
+class StackFrame;
 class Error;
 class Function;
 class StdlibFunction;
@@ -100,7 +100,7 @@ class Object {
   virtual Result<short> cmp(const char* rhs) const {
     return ERROR(NotImplemented);
   }
-  virtual Result<short> cmp(const Stack& rhs) const {
+  virtual Result<short> cmp(const StackFrame& rhs) const {
     return ERROR(NotImplemented);
   }
   virtual Result<short> cmp(const Error& rhs) const {
@@ -1086,23 +1086,25 @@ class Module : public Object {
   GcRoot<PodModule> _value;
 };
 
-class Stack : public Object {
+class StackFrame : public Object {
  public:
-  Stack() {}
-  Stack(Stack&& rhs) : _value(std::move(rhs._value)) {}
-  Stack(GcRoot<PodStack>&& val) : _value(std::move(val)) {}
+  StackFrame() {}
+  StackFrame(StackFrame&& rhs) : _value(std::move(rhs._value)) {}
+  StackFrame(GcRoot<PodStackFrame>&& val) : _value(std::move(val)) {}
 
-  virtual Tag tag() const final { return Tag::Stack; }
+  virtual Tag tag() const final { return Tag::StackFrame; }
   virtual PodObject* pod() const final { return _value.get(); }
   virtual Result<short> cmp(const Object& rhs) const final {
     return -TRY(rhs.cmp(*this));
   }
-  virtual Result<short> cmp(const Stack& rhs) const final { return 0; }
+  virtual Result<short> cmp(const StackFrame& rhs) const final { return 0; }
 
-  virtual void move(Object* obj) final { new (obj) Stack(std::move(_value)); }
+  virtual void move(Object* obj) final {
+    new (obj) StackFrame(std::move(_value));
+  }
 
-  static Result<Stack> create(PodStack* obj) {
-    return Stack(TRY(MkGcRoot(obj)));
+  static Result<StackFrame> create(PodStackFrame* obj) {
+    return StackFrame(TRY(MkGcRoot(obj)));
   }
 
   using Object::get;
@@ -1111,19 +1113,7 @@ class Stack : public Object {
   Result<void> settop(uint64_t idx);
   uint64_t gettop() { return _value->top; }
 
-  static Result<Stack> create(uint64_t size) {
-    auto pod = TRY(arena_alloc<PodStack>(sizeof(OffPtr<PodObject>) * size));
-    pod->header.tag = Tag::Stack;
-
-    pod->size = size;
-    pod->top = 0;
-
-    for (uint64_t i = 0; i < size; i++) {
-      pod->data[i] = 0;
-    }
-
-    return Stack(TRY(MkGcRoot(pod)));
-  }
+  static Result<StackFrame> create(uint64_t size);
 
   Result<Array> slice(uint64_t start, uint64_t end) {
     if (start > end || end > gettop()) return ERROR(IndexOutOfRange);
@@ -1139,10 +1129,10 @@ class Stack : public Object {
   }
 
   virtual Result<Value> copy_value() const final;
-  Result<Stack> copy() const;
+  Result<StackFrame> copy() const;
 
  private:
-  GcRoot<PodStack> _value;
+  GcRoot<PodStackFrame> _value;
 };
 
 class Error : public Object {
diff --git a/src/compiler.cpp b/src/compiler.cpp
index a06425f..91fc31c 100644
--- a/src/compiler.cpp
+++ b/src/compiler.cpp
@@ -237,7 +237,7 @@ Result<Expression> Compiler::compile_expr(Context& context, const Value& expr) {
     case Tag::Function:
     case Tag::StdlibFunction:
     case Tag::Module:
-    case Tag::Stack:
+    case Tag::StackFrame:
     case Tag::Error:
       return ERROR(TypeMismatch);
   }
diff --git a/src/pod.hpp b/src/pod.hpp
index 8432ce9..94f4fdf 100644
--- a/src/pod.hpp
+++ b/src/pod.hpp
@@ -23,7 +23,7 @@ enum class Tag : uint8_t {
   Function,
   StdlibFunction,
   Module,
-  Stack,
+  StackFrame,
   Error,
 };
 
@@ -192,10 +192,11 @@ class PodModule final : public PodObject {
   OffPtr<PodObject> fun;
 };
 
-class PodStack final : public PodObject {
+class PodStackFrame final : public PodObject {
  public:
-  PodStack() : PodObject(Tag::Stack) {};
+  PodStackFrame() : PodObject(Tag::StackFrame) {};
 
+  OffPtr<PodObject> parent;
   uint64_t size;
   uint64_t top;
   OffPtr<PodObject> data[];
diff --git a/src/vm.hpp b/src/vm.hpp
index 9be2906..17ecf1c 100644
--- a/src/vm.hpp
+++ b/src/vm.hpp
@@ -5,15 +5,15 @@
 class VM {
  public:
   VM() {}
-  VM(Stack&& stack, Stack&& callstack)
+  VM(StackFrame&& stack, StackFrame&& callstack)
       : _stack(std::move(stack)), _callstack(std::move(callstack)) {}
   VM(VM&& vm)
       : _stack(std::move(vm._stack)), _callstack(std::move(vm._callstack)) {}
   VM(const VM&) = delete;
 
   static Result<VM> create() {
-    auto stack = TRY(Stack::create(16 * 1024));
-    auto callstack = TRY(Stack::create(16 * 1024));
+    auto stack = TRY(StackFrame::create(16 * 1024));
+    auto callstack = TRY(StackFrame::create(16 * 1024));
     return VM(std::move(stack), std::move(callstack));
   }
 
@@ -48,13 +48,11 @@ class VM {
   Result<Value> getreg(uint64_t idx);
   Result<void> setreg(uint64_t idx, const Value& value);
 
-  Result<Dict> globals() {
-    return _globals.copy();
-  }
+  Result<Dict> globals() { return _globals.copy(); }
 
  private:
-  Stack _stack;
-  Stack _callstack;
+  StackFrame _stack;
+  StackFrame _callstack;
   Function _fun;
   Array _code;
   Array _constants;
diff --git a/src/writer.cpp b/src/writer.cpp
index f091ad9..713a468 100644
--- a/src/writer.cpp
+++ b/src/writer.cpp
@@ -39,8 +39,8 @@ Result<String> Writer::write_one(const Value& obj) {
       return write_stdlib_function(*obj.to<StdlibFunction>());
     case Tag::Module:
       return write_module(*obj.to<Module>());
-    case Tag::Stack:
-      return write_stack(*obj.to<Stack>());
+    case Tag::StackFrame:
+      return write_stack(*obj.to<StackFrame>());
     case Tag::Error:
       return write_error(*obj.to<Error>());
   };
@@ -376,7 +376,7 @@ Result<String> Writer::write_module(const Module& val) {
   return res;
 }
 
-Result<String> Writer::write_stack(const Stack& val) {
+Result<String> Writer::write_stack(const StackFrame& val) {
   return TRY(String::create("#<stack>"));
 }
 
diff --git a/src/writer.hpp b/src/writer.hpp
index bc79004..2f3df43 100644
--- a/src/writer.hpp
+++ b/src/writer.hpp
@@ -27,7 +27,7 @@ class Writer {
   Result<String> write_function(const Function& val);
   Result<String> write_stdlib_function(const StdlibFunction& val);
   Result<String> write_module(const Module& val);
-  Result<String> write_stack(const Stack& val);
+  Result<String> write_stack(const StackFrame& val);
   Result<String> write_error(const Error& val);
 };