Index: Lib/idlelib/EditorWindow.py
===================================================================
--- Lib/idlelib/EditorWindow.py	(revision 68230)
+++ Lib/idlelib/EditorWindow.py	(working copy)
@@ -315,6 +315,9 @@
 
             first = self.text.index(dest)
             last = self.text.index("anchor")
+            if not last:
+                # The 'anchor' mark didn't exist up to this point.
+                last = self.text.index("sel.last")
 
             if self.text.compare(first,">",last):
                 first,last = last,first