diff --git a/tools/test.py b/tools/test.py
index cdeb40d59c..c3a7ff07e2 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -375,7 +375,10 @@ class TapProgressIndicator(SimpleProgressIndicator):
 
       if output.diagnostic:
         self.severity = 'ok'
-        self.traceback = output.diagnostic
+        if isinstance(output.diagnostic, list):
+          self.traceback = '\n'.join(output.diagnostic)
+        else:
+          self.traceback = output.diagnostic
 
 
     duration = output.test.duration