diff --git a/Lib/idlelib/EditorWindow.py b/Lib/idlelib/EditorWindow.py
index f16badb..1230047 100644
--- a/Lib/idlelib/EditorWindow.py
+++ b/Lib/idlelib/EditorWindow.py
@@ -1135,7 +1135,10 @@
         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: