diff --git a/reporting.py b/reporting.py
index 63ac926e..579aa211 100644
--- a/reporting.py
+++ b/reporting.py
@@ -33,6 +33,8 @@ def to_json_serializable(o):
         return tuple(to_json_serializable(i) for i in o)
     if isinstance(o, list):
         return [to_json_serializable(i) for i in o]
+    if callable(o):
+        return repr(o)
 
     # Ensure everything is JSON serializable. If this warning is issued, it
     # means the given type needs to be added above if possible.