diff --git a/imperative/python/megengine/autodiff/grad_manager.py b/imperative/python/megengine/autodiff/grad_manager.py
index 3d569795941273d25910c7c64e4a61efa20e93ad..72c689fd4453bcb4143d6955dacc1eef8c0a77f4 100644
--- a/imperative/python/megengine/autodiff/grad_manager.py
+++ b/imperative/python/megengine/autodiff/grad_manager.py
@@ -1,7 +1,7 @@
 import weakref
 from collections import defaultdict
 from contextlib import contextmanager
-from typing import Callable
+from typing import Callable, Iterable
 
 from ..core._imperative_rt.core2 import pop_scope, push_scope, set_option
 from ..core.autodiff.grad import Grad
@@ -121,7 +121,7 @@ class GradManager:
         self._after_backward_callback = []
         self._gradients = {}
 
-    def attach(self, tensors: list, callbacks=None):
+    def attach(self, tensors: Iterable[Tensor], callbacks=None):
         r"""
         Instruct GradManager to track operations on tensors, so that gradients with respect
         to those tensors could be evaluated later.
@@ -199,6 +199,7 @@ class GradManager:
             return spec
 
         for x in tensors:
+            assert isinstance(x, Tensor), "Object to be attached should be Tensor"
             spec = self._attach_specs.get(id(x))
             new_attach = spec is None
             if spec is None: