diff --git a/sam3/model/encoder.py b/sam3/model/encoder.py
index 3fc94061..3465db9e 100644
--- a/sam3/model/encoder.py
+++ b/sam3/model/encoder.py
@@ -538,7 +538,7 @@ def forward(
                     else None
                 )
         else:
-            assert all(x.dim == 4 for x in src), (
+            assert all(x.ndim == 4 for x in src), (
                 "expected list of (bs, c, h, w) tensors"
             )