diff --git a/mcode.cm b/mcode.cm
index 8e914d14..163c1073 100644
--- a/mcode.cm
+++ b/mcode.cm
@@ -706,9 +706,6 @@ var mcode = function(ast) {
     var argc = length(args)
     var frame_slot = alloc_slot()
     emit_3("frame", frame_slot, func_slot, argc)
-    var null_slot = alloc_slot()
-    emit_1("null", null_slot)
-    emit_3("setarg", frame_slot, 0, null_slot)
     var arg_idx = 1
     var _i = 0
     while (_i < argc) {
diff --git a/source/mach.c b/source/mach.c
index 5d921d8e..6d42aa6f 100644
--- a/source/mach.c
+++ b/source/mach.c
@@ -2567,7 +2567,15 @@ vm_dispatch:
         frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
         goto disrupt;
       }
-      int nr = c + 2; /* argc + this + func overhead */
+      JSFunction *fn = JS_VALUE_GET_FUNCTION(func_val);
+      int nr;
+      if (fn->kind == JS_FUNC_KIND_REGISTER) {
+        JSCodeRegister *fn_code = JS_VALUE_GET_CODE(fn->u.cell.code)->u.reg.code;
+        nr = fn_code->nr_slots;
+        if (nr < c + 2) nr = c + 2; /* safety: never smaller than argc+2 */
+      } else {
+        nr = c + 2;
+      }
       JSFrameRegister *call_frame = alloc_frame_register(ctx, nr);
       if (!call_frame) {
         frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
@@ -2576,6 +2584,7 @@ vm_dispatch:
       frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
       func_val = frame->slots[b]; /* re-read after GC */
       call_frame->function = func_val;
+      call_frame->address = JS_NewInt32(ctx, c); /* store actual argc */
       frame->slots[a] = JS_MKPTR(call_frame);
       VM_BREAK();
     }
@@ -2588,36 +2597,19 @@ vm_dispatch:
     VM_CASE(MACH_INVOKE): {
       /* A=frame_slot, B=result_slot */
       JSFrameRegister *fr = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->slots[a]);
-      int nr = (int)objhdr_cap56(fr->header);
-      int c_argc = (nr >= 2) ? nr - 2 : 0;
+      int c_argc = JS_VALUE_GET_INT(fr->address); /* actual argc stored by FRAME */
       JSValue fn_val = fr->function;
       JSFunction *fn = JS_VALUE_GET_FUNCTION(fn_val);
       if (!mach_check_call_arity(ctx, fn, c_argc))
         goto disrupt;
 
       if (fn->kind == JS_FUNC_KIND_REGISTER) {
-        /* Register function: switch frames inline (fast path) */
+        /* Register function: FRAME already allocated nr_slots — just switch */
         JSCodeRegister *fn_code = JS_VALUE_GET_CODE(fn->u.cell.code)->u.reg.code;
-        JSFrameRegister *new_frame = alloc_frame_register(ctx, fn_code->nr_slots);
-        if (!new_frame) {
-          frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
-          goto disrupt;
-        }
-        frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
-        fr = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->slots[a]);
-        fn_val = fr->function;
-        fn = JS_VALUE_GET_FUNCTION(fn_val);
-        fn_code = JS_VALUE_GET_CODE(fn->u.cell.code)->u.reg.code;
-        new_frame->function = fn_val;
-        /* Copy this + args from call frame to new frame */
-        int copy_count = (c_argc < fn_code->arity) ? c_argc : fn_code->arity;
-        new_frame->slots[0] = fr->slots[0]; /* this */
-        for (int i = 0; i < copy_count; i++)
-          new_frame->slots[1 + i] = fr->slots[1 + i];
         /* Save return info */
         frame->address = JS_NewInt32(ctx, (pc << 16) | b);
-        new_frame->caller = JS_MKPTR(frame);
-        frame = new_frame;
+        fr->caller = JS_MKPTR(frame);
+        frame = fr;
         frame_ref.val = JS_MKPTR(frame);
         code = fn_code;
         env = fn->u.cell.env_record;
@@ -2661,8 +2653,7 @@ vm_dispatch:
     VM_CASE(MACH_GOINVOKE): {
       /* Tail call: replace current frame with callee */
       JSFrameRegister *fr = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->slots[a]);
-      int nr = (int)objhdr_cap56(fr->header);
-      int c_argc = (nr >= 2) ? nr - 2 : 0;
+      int c_argc = JS_VALUE_GET_INT(fr->address); /* actual argc stored by FRAME */
       JSValue fn_val = fr->function;
       JSFunction *fn = JS_VALUE_GET_FUNCTION(fn_val);
       if (!mach_check_call_arity(ctx, fn, c_argc))
@@ -2687,25 +2678,10 @@ vm_dispatch:
           env = fn->u.cell.env_record;
           pc = code->entry_point;
         } else {
-          /* SLOW PATH: callee needs more slots, must allocate */
-          JSFrameRegister *new_frame = alloc_frame_register(ctx, fn_code->nr_slots);
-          if (!new_frame) {
-            frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
-            goto disrupt;
-          }
-          frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
-          fr = (JSFrameRegister *)JS_VALUE_GET_PTR(frame->slots[a]);
-          fn_val = fr->function;
-          fn = JS_VALUE_GET_FUNCTION(fn_val);
-          fn_code = JS_VALUE_GET_CODE(fn->u.cell.code)->u.reg.code;
-          new_frame->function = fn_val;
-          int copy_count = (c_argc < fn_code->arity) ? c_argc : fn_code->arity;
-          new_frame->slots[0] = fr->slots[0]; /* this */
-          for (int i = 0; i < copy_count; i++)
-            new_frame->slots[1 + i] = fr->slots[1 + i];
-          new_frame->caller = frame->caller;
+          /* SLOW PATH: GOFRAME already allocated nr_slots — use fr directly */
+          fr->caller = frame->caller;
           frame->caller = JS_NULL;
-          frame = new_frame;
+          frame = fr;
           frame_ref.val = JS_MKPTR(frame);
           code = fn_code;
           env = fn->u.cell.env_record;