Index: AutoCompleteWindow.py
===================================================================
--- AutoCompleteWindow.py	(revision 60721)
+++ AutoCompleteWindow.py	(working copy)
@@ -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.