diff --git a/gwinc/nb.py b/gwinc/nb.py
index 310db63fa7c3bf62fd11cc84cb6e13d749d3e596..92ade0c57efdf7da64db4a074a3ea518382bb092 100644
--- a/gwinc/nb.py
+++ b/gwinc/nb.py
@@ -78,7 +78,7 @@ class BudgetItem:
     def __str__(self):
         # FIXME: provide info on internal state (load/update/calc/etc.)
         return '<{} {}>'.format(
-            ' '.join([c.__name__ for c in self.__class__.__bases__]),
+            ', '.join([c.__name__ for c in self.__class__.__bases__]),
             self.name,
         )