diff --git a/Lib/idlelib/OutputWindow.py b/Lib/idlelib/OutputWindow.py
index 0e8fd6db0a8..83d5771d7e0 100644
--- a/Lib/idlelib/OutputWindow.py
+++ b/Lib/idlelib/OutputWindow.py
@@ -35,7 +35,7 @@ class OutputWindow(EditorWindow):
     # Act as output file
 
     def write(self, s, tags=(), mark="insert"):
-        assert isinstance(s, str)
+        assert isinstance(s, str), repr(s)
         self.text.insert(mark, s, tags)
         self.text.see(mark)
         self.text.update()