diff --git a/src/njs_vmcode.c b/src/njs_vmcode.c
--- a/src/njs_vmcode.c
+++ b/src/njs_vmcode.c
@@ -1218,9 +1218,16 @@ static njs_jump_off_t
 njs_vmcode_function_copy(njs_vm_t *vm, njs_value_t *value, njs_index_t retidx)
 {
     njs_value_t     *retval;
     njs_function_t  *function;
+    njs_bool_t       fncopy_dbg_invalid;
+
+    fncopy_dbg_invalid = !njs_is_valid(njs_scope_value(vm, retidx));

     retval = njs_scope_valid_value(vm, retidx);

     if (njs_is_undefined(retval)) {
+        if (njs_slow_path(!fncopy_dbg_invalid)) {
+            njs_stderror("FNCOPY-INVARIANT re-init of assigned binding\n");
+            abort();
+        }
         *retval = *value;