Addendum to #764548: restore 2.1 compatibility.
diff --git a/Lib/sre.py b/Lib/sre.py
index 7910c83..ffe2bc3 100644
--- a/Lib/sre.py
+++ b/Lib/sre.py
@@ -221,7 +221,7 @@
     pattern, flags = key
     if isinstance(pattern, _pattern_type):
         return pattern
-    if not isinstance(pattern, sre_compile.STRING_TYPES):
+    if not sre_compile.isstring(pattern):
         raise TypeError, "first argument must be string or compiled pattern"
     try:
         p = sre_compile.compile(pattern, flags)