diff --git a/tests/plugins.py b/tests/plugins.py
index 90c5c0d99..4e7af97ae 100644
--- a/tests/plugins.py
+++ b/tests/plugins.py
@@ -97,6 +97,8 @@ def finalize(self, result):
 
     def formatErr(self, err):
         exctype, value, tb = err
+        if not isinstance(value, exctype):
+            value = exctype(value)
         return ''.join(traceback.format_exception(exctype, value, tb))
 
     def startContext(self, ctx):