diff --git a/torchdrug/data/molecule.py b/torchdrug/data/molecule.py
index edf5406..8374da5 100644
--- a/torchdrug/data/molecule.py
+++ b/torchdrug/data/molecule.py
@@ -518,7 +518,7 @@ def is_valid(self):
         """A coarse implementation of valence check."""
         # TODO: cross-check by any domain expert
         atom2valence = torch.tensor(float("nan")).repeat(constant.NUM_ATOM)
-        for k, v in self.atom2valence:
+        for k, v in self.atom2valence.items():
             atom2valence[k] = v
         atom2valence = torch.as_tensor(atom2valence, device=self.device)