diff --git a/src/torchio/data/queue.py b/src/torchio/data/queue.py
index c0090881..a2034ee3 100644
--- a/src/torchio/data/queue.py
+++ b/src/torchio/data/queue.py
@@ -258,7 +258,7 @@ def subjects_iterable(self):
     @property
     def num_subjects(self) -> int:
         if self.subject_sampler is not None:
-            return len(self.subject_sampler)
+            return self.subject_sampler.__len__()
         return len(self.subjects_dataset)
 
     @property