diff --git a/Lib/urllib/request.py b/Lib/urllib/request.py
index e3eed16..b097dd3 100644
--- a/Lib/urllib/request.py
+++ b/Lib/urllib/request.py
@@ -1170,7 +1170,8 @@ class AbstractDigestAuthHandler:
             H = lambda x: hashlib.md5(x.encode("ascii")).hexdigest()
         elif algorithm == 'SHA':
             H = lambda x: hashlib.sha1(x.encode("ascii")).hexdigest()
-        # XXX MD5-sess
+        else: # XXX MD5-sess
+            raise NotImplementedError("Unsupported Digest Authentication Algorithm %r" % algorithm)
         KD = lambda s, d: H("%s:%s" % (s, d))
         return H, KD