diff -r 9468b094ae2b -r 0b600abb3074 Lib/idlelib/EditorWindow.py
--- a/Lib/idlelib/EditorWindow.py	Fri Oct 14 10:30:24 2011 -0500
+++ b/Lib/idlelib/EditorWindow.py	Fri Oct 14 11:34:29 2011 -0500
@@ -1129,7 +1129,10 @@
         assert have > 0
         want = ((have - 1) // self.indentwidth) * self.indentwidth
         # Debug prompt is multilined....
-        last_line_of_prompt = sys.ps1.split('\n')[-1]
+        if self.context_use_ps1:
+            last_line_of_prompt = sys.ps1.split('\n')[-1]
+        else:
+            last_line_of_prompt = ''
         ncharsdeleted = 0
         while 1:
             if chars == last_line_of_prompt: