diff --git a/include/pocketpy/vm.h b/include/pocketpy/vm.h
index 6363b641..2f96f763 100644
--- a/include/pocketpy/vm.h
+++ b/include/pocketpy/vm.h
@@ -309,7 +309,8 @@ public:
     }
 
     i64 normalized_index(i64 index, int size);
-    PyObject* py_next(PyObject* obj);
+    PyObject* py_next(PyObject*);
+    PyObject* _py_next(const PyTypeInfo*, PyObject*);
     PyObject* _pack_next_retval(unsigned);
     bool py_callable(PyObject* obj);
     
diff --git a/src/ceval.cpp b/src/ceval.cpp
index 741c1c89..d6026e25 100644
--- a/src/ceval.cpp
+++ b/src/ceval.cpp
@@ -48,12 +48,13 @@ void VM::_op_unpack_sequence(uint16_t arg){
     }else{
         auto _lock = heap.gc_scope_lock();  // lock the gc via RAII!!
         _0 = py_iter(_0);
+        const PyTypeInfo* ti = _inst_type_info(_0);
         for(int i=0; i<arg; i++){
-            PyObject* _1 = py_next(_0);
+            PyObject* _1 = _py_next(ti, _0);
             if(_1 == StopIteration) ValueError("not enough values to unpack");
             PUSH(_1);
         }
-        if(py_next(_0) != StopIteration) ValueError("too many values to unpack");
+        if(_py_next(ti, _0) != StopIteration) ValueError("too many values to unpack");
     }
 }
 
@@ -850,15 +851,16 @@ __NEXT_STEP:;
     TARGET(UNPACK_EX) {
         auto _lock = heap.gc_scope_lock();  // lock the gc via RAII!!
         PyObject* _0 = py_iter(POPX());
+        const PyTypeInfo* _ti = _inst_type_info(_0);
         PyObject* _1;
         for(int i=0; i<byte.arg; i++){
-            _1 = py_next(_0);
+            _1 = _py_next(_ti, _0);
             if(_1 == StopIteration) ValueError("not enough values to unpack");
             PUSH(_1);
         }
         List extras;
         while(true){
-            _1 = py_next(_0);
+            _1 = _py_next(_ti, _0);
             if(_1 == StopIteration) break;
             extras.push_back(_1);
         }
diff --git a/src/pocketpy.cpp b/src/pocketpy.cpp
index d3ca8281..08c50f0c 100644
--- a/src/pocketpy.cpp
+++ b/src/pocketpy.cpp
@@ -683,11 +683,12 @@ void init_builtins(VM* _vm) {
         const Str& self = _CAST(Str&, args[0]);
         SStream ss;
         PyObject* it = vm->py_iter(args[1]);     // strong ref
-        PyObject* obj = vm->py_next(it);
+        const PyTypeInfo* info = vm->_inst_type_info(args[1]);
+        PyObject* obj = vm->_py_next(info, it);
         while(obj != vm->StopIteration){
             if(!ss.empty()) ss << self;
             ss << CAST(Str&, obj);
-            obj = vm->py_next(it);
+            obj = vm->_py_next(info, it);
         }
         return VAR(ss.str());
     });
@@ -904,10 +905,11 @@ void init_builtins(VM* _vm) {
         auto _lock = vm->heap.gc_scope_lock();
         List& self = _CAST(List&, args[0]);
         PyObject* it = vm->py_iter(args[1]);     // strong ref
-        PyObject* obj = vm->py_next(it);
+        const PyTypeInfo* info = vm->_inst_type_info(args[1]);
+        PyObject* obj = vm->_py_next(info, it);
         while(obj != vm->StopIteration){
             self.push_back(obj);
-            obj = vm->py_next(it);
+            obj = vm->_py_next(info, it);
         }
         return vm->None;
     });
diff --git a/src/vm.cpp b/src/vm.cpp
index 7dce806c..fcb38dec 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -250,8 +250,7 @@ namespace pkpy{
         return index;
     }
 
-    PyObject* VM::py_next(PyObject* obj){
-        const PyTypeInfo* ti = _inst_type_info(obj);
+    PyObject* VM::_py_next(const PyTypeInfo* ti, PyObject* obj){
         if(ti->m__next__){
             unsigned n = ti->m__next__(this, obj);
             return _pack_next_retval(n);
@@ -259,6 +258,11 @@ namespace pkpy{
         return call_method(obj, __next__);
     }
 
+    PyObject* VM::py_next(PyObject* obj){
+        const PyTypeInfo* ti = _inst_type_info(obj);
+        return _py_next(ti, obj);
+    }
+
     bool VM::py_callable(PyObject* obj){
         Type cls = vm->_tp(obj);
         switch(cls.index){
@@ -381,10 +385,11 @@ PyObject* VM::py_list(PyObject* it){
     auto _lock = heap.gc_scope_lock();
     it = py_iter(it);
     List list;
-    PyObject* obj = py_next(it);
+    const PyTypeInfo* info = _inst_type_info(it);
+    PyObject* obj = _py_next(info, it);
     while(obj != StopIteration){
         list.push_back(obj);
-        obj = py_next(it);
+        obj = _py_next(info, it);
     }
     return VAR(std::move(list));
 }
@@ -771,10 +776,11 @@ void VM::_unpack_as_list(ArgsView args, List& list){
             // maybe this check should be done in the compile time
             if(w.level != 1) TypeError("expected level 1 star wrapper");
             PyObject* _0 = py_iter(w.obj);
-            PyObject* _1 = py_next(_0);
+            const PyTypeInfo* info = _inst_type_info(_0);
+            PyObject* _1 = _py_next(info, _0);
             while(_1 != StopIteration){
                 list.push_back(_1);
-                _1 = py_next(_0);
+                _1 = _py_next(info, _0);
             }
         }else{
             list.push_back(obj);
diff --git a/tests/99_builtin_func.py b/tests/99_builtin_func.py
index 084830a3..fddfd44b 100644
--- a/tests/99_builtin_func.py
+++ b/tests/99_builtin_func.py
@@ -268,17 +268,6 @@ assert type(12 * [12]) is list
 
 
 # /************ tuple ************/
-# 未完全测试准确性-----------------------------------------------
-#       180:  783:    _vm->bind_constructor<-1>("tuple", [](VM* vm, ArgsView args) {
-#        32:  784:        if(args.size() == 1+0) return VAR(Tuple(0));
-#        32:  785:        if(args.size() == 1+1){
-#        32:  786:            List list = CAST(List, vm->py_list(args[1]));
-#        32:  787:            return VAR(Tuple(std::move(list)));
-#        32:  788:        }
-#     #####:  789:        vm->TypeError("tuple() takes at most 1 argument");
-#     #####:  790:        return vm->None;
-#        32:  791:    });
-#         -:  792:
 # test tuple:
 try:
     tuple(1,2)