diff --git a/km3io/online.py b/km3io/online.py
index 59e2fee3beb4f5805f4e081de637bac4e91375c6..5aa9774bacc964e7120b97c6066595125730e2d2 100644
--- a/km3io/online.py
+++ b/km3io/online.py
@@ -61,7 +61,7 @@ class SummarysliceReader:
         return next(self._summaryslices)
 
     def __len__(self):
-        return int(np.ceil(len(self._branch) / self._step_size))
+        return int(np.ceil(self._branch.num_entries / self._step_size))
 
 
 @nb.vectorize(