diff --git a/source/mach.c b/source/mach.c
index 93341ac9..61bd67f3 100644
--- a/source/mach.c
+++ b/source/mach.c
@@ -494,6 +494,37 @@ static int mach_check_call_arity(JSContext *ctx, JSFunction *fn, int argc) {
   return 1;
 }
 
+/* Scan backwards from pc to find what loaded the callee register.
+   Returns the name in buf, or NULL if unknown. */
+static const char *mach_callee_name(JSContext *ctx, JSCodeRegister *code,
+                                    uint32_t pc, int reg,
+                                    char *buf, size_t buf_size) {
+  int hops = 4; /* limit move-chain depth */
+  for (int i = (int)pc - 2; i >= 0 && hops > 0; i--) {
+    MachInstr32 prev = code->instructions[i];
+    int prev_op = MACH_GET_OP(prev);
+    int prev_a = MACH_GET_A(prev);
+    if (prev_a != reg) continue;
+    if (prev_op == MACH_GETENV || prev_op == MACH_GETINTRINSIC) {
+      int bx = MACH_GET_Bx(prev);
+      if ((uint32_t)bx < code->cpool_count && JS_IsText(code->cpool[bx]))
+        return JS_KeyGetStr(ctx, buf, buf_size, code->cpool[bx]);
+    }
+    if (prev_op == MACH_LOAD_FIELD) {
+      int ci = MACH_GET_C(prev);
+      if ((uint32_t)ci < code->cpool_count && JS_IsText(code->cpool[ci]))
+        return JS_KeyGetStr(ctx, buf, buf_size, code->cpool[ci]);
+    }
+    if (prev_op == MACH_MOVE) {
+      reg = MACH_GET_B(prev);
+      hops--;
+      continue;
+    }
+    break; /* some other op wrote to this reg — give up */
+  }
+  return NULL;
+}
+
 /* ---- Link pass: resolve GETNAME to GETINTRINSIC or GETENV ---- */
 
 static void mach_link_code(JSContext *ctx, JSCodeRegister *code, JSValue env) {
@@ -2560,7 +2591,12 @@ vm_dispatch:
       /* A=frame_slot, B=func_reg, C=argc */
       JSValue func_val = frame->slots[b];
       if (!mist_is_function(func_val)) {
-        JS_RaiseDisrupt(ctx, "not a function");
+        char nbuf[KEY_GET_STR_BUF_SIZE];
+        const char *name = mach_callee_name(ctx, code, pc, b, nbuf, sizeof(nbuf));
+        if (name)
+          JS_RaiseDisrupt(ctx, "%s is not a function", name);
+        else
+          JS_RaiseDisrupt(ctx, "not a function");
         frame = (JSFrameRegister *)JS_VALUE_GET_PTR(frame_ref.val);
         goto disrupt;
       }