diff --git a/km3io/rootio.py b/km3io/rootio.py
index a5e48564cb524652819c7fc08107259c558a681f..75e643632f02f6d60b312b683863d6a7a29c635b 100644
--- a/km3io/rootio.py
+++ b/km3io/rootio.py
@@ -174,7 +174,7 @@ class Branch:
     def __len__(self):
         if not self._index_chain:
             return len(self._branch)
-        elif isinstance(self._index_chain[-1], (int, np.int32, np.int64)):
+        elif isinstance(self._index_chain[0], (int, np.int32, np.int64)):
             if len(self._index_chain) == 1:
                 try:
                     return len(self[:])