diff --git a/km3io/rootio.py b/km3io/rootio.py
index ffaf69f0f66b7b9f4123a0da935a85620d7ca6b7..34f0b68d3740a4034e69197f8b170a2c1d8400cb 100644
--- a/km3io/rootio.py
+++ b/km3io/rootio.py
@@ -171,7 +171,7 @@ class Branch:
     def __len__(self):
         if not self._index_chain:
             return len(self._branch)
-        elif isinstance(self._index_chain[-1], int):
+        elif isinstance(self._index_chain[-1], (int, np.int32, np.int64)):
             return 1
         else:
             return len(