diff --git a/tool/lib/test/unit/core_assertions.rb b/tool/lib/test/unit/core_assertions.rb
index 63d79ae2aa..b2566561ce 100644
--- a/tool/lib/test/unit/core_assertions.rb
+++ b/tool/lib/test/unit/core_assertions.rb
@@ -216,7 +216,7 @@ eom
           }
 
           assert expected, proc {
-            exception_details(e, message(msg) {"#{mu_pp(exp)} exception expected, not"}.call)
+            flunk(message(msg) {"#{mu_pp(exp)} exception expected, not #{mu_pp(e)}"})
           }
 
           return e