diff --git a/torch_frame/data/mapper.py b/torch_frame/data/mapper.py
index 3c388f79..c019b364 100644
--- a/torch_frame/data/mapper.py
+++ b/torch_frame/data/mapper.py
@@ -110,7 +110,7 @@ def forward(
 
     def backward(self, tensor: Tensor) -> pd.Series:
         index = tensor.cpu().numpy()
-        ser = pd.Series(self.categories[index].index)
+        ser = pd.Series(self.categories.iloc[index].index)
         ser[index < 0] = None
         return ser