diff --git a/ibllib/io/extractors/mesoscope.py b/ibllib/io/extractors/mesoscope.py
index 5f915cf82..b95da0257 100644
--- a/ibllib/io/extractors/mesoscope.py
+++ b/ibllib/io/extractors/mesoscope.py
@@ -759,7 +759,7 @@ def get_bout_edges(self, frame_times, collections=None, events=None, min_gap=1.,
                 include = sorted(int(c.rsplit('_', 1)[-1]) for c in collections)
                 edges = edges[include, :]
             elif edges.shape[0] < len(collections):
-                raise ValueError('More raw imaging folders than detected bouts')
+                raise ValueError(f'More raw imaging folders (={len(collections)}) than detected bouts (={edges.shape[0]})')
 
         if display:
             _, ax = plt.subplots(1)