Index: AutoCompleteWindow.py
===================================================================
--- AutoCompleteWindow.py	(revision 60721)
+++ AutoCompleteWindow.py	(working copy)
@@ -55,8 +55,8 @@
 
     def _change_start(self, newstart):
         i = 0
-        while i < len(self.start) and i < len(newstart) and \
-              self.start[i] == newstart[i]:
+        min_len = min(len(self.start), len(newstart))
+        while i < min_len and self.start[i] == newstart[i]:
             i += 1
         if i < len(self.start):
             self.widget.delete("%s+%dc" % (self.startindex, i),
@@ -98,13 +98,17 @@
                 i = m + 1
         last = i-1
 
+        if first == last: # There is only one possible completion - return it
+            return self.completions[first]
+
         # We should return the maximum prefix of first and last
-        i = len(s)
-        while len(self.completions[first]) > i and \
-              len(self.completions[last]) > i and \
-              self.completions[first][i] == self.completions[last][i]:
+        i = len(s) + 1
+        first_comp = self.completions[first]
+        last_comp = self.completions[last]
+        min_len = min(len(first_comp), len(last_comp))
+        while i < min_len and first_comp[i] == last_comp[i]:
             i += 1
-        return self.completions[first][:i]
+        return first_comp[:i]
 
     def _selection_changed(self):
         """Should be called when the selection of the Listbox has changed.
@@ -119,7 +123,8 @@
             newstart = lts
         else:
             i = 0
-            while i < len(lts) and i < len(selstart) and lts[i] == selstart[i]:
+            min_len = min(len(lts), len(selstart))
+            while i < min_len and lts[i] == selstart[i]:
                 i += 1
             newstart = selstart[:i]
         self._change_start(newstart)