diff --git a/src/torchio/data/queue.py b/src/torchio/data/queue.py
index a9869ee1..7f6b5872 100644
--- a/src/torchio/data/queue.py
+++ b/src/torchio/data/queue.py
@@ -263,7 +263,7 @@ def num_subjects(self) -> int:
                     'The subject sampler passed to the queue must have a'
                     ' __len__ method',
                 )
-            num_subjects = len(self.subject_sampler)
+            num_subjects = len(self.subject_sampler)  # type: ignore[arg-type]
         else:
             num_subjects = len(self.subjects_dataset)
         return num_subjects