diff --git a/ami/main/models.py b/ami/main/models.py
index e144a82f0..1f5c64f7a 100644
--- a/ami/main/models.py
+++ b/ami/main/models.py
@@ -515,7 +515,7 @@ def duration_label(self) -> str:
         If duration was populated by a query annotation, use that
         otherwise call the duration() method to calculate it.
         """
-        duration = self.duration if isinstance(self.duration, datetime.timedelta) else self.duration()
+        duration = self.duration() if callable(self.duration) else self.duration
         return ami.utils.dates.format_timedelta(duration)
 
     # These are now loaded with annotations in EventViewSet
@@ -1475,7 +1475,7 @@ def duration_label(self) -> str | None:
         If duration has been calculated by a query annotation, use that value
         otherwise call the duration() method to calculate it.
         """
-        duration = self.duration if isinstance(self.duration, datetime.timedelta) else self.duration()
+        duration = self.duration() if callable(self.duration) else self.duration
         return ami.utils.dates.format_timedelta(duration)
 
     def detection_images(self, limit=None):