diff --git a/setup.cfg b/setup.cfg
index 2fded5b29..4e3c6b40c 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -4,5 +4,14 @@ cover-html=true
 cover-erase=true
 cover-inclusive=true
 verbosity=2
-detailed-errors=1
+
+; Disabled: Causes a bug in testtools to manifest.
+; Trigger: self.assertX(condition), where condition == False.
+;
+; In "testtools/testresult/real.py" the traceback is set to
+; None in _details_to_exc_info(), but the inspect_traceback()
+; method in nose/inspector.py requires a traceback-like object.
+;
+; detailed-errors=1
+
 where=marconi/tests