diff --git a/requests/auth.py b/requests/auth.py
index a3de123d..48416e5f 100644
--- a/requests/auth.py
+++ b/requests/auth.py
@@ -144,7 +144,7 @@ class HTTPDigestAuth(AuthBase):
         if entdig:
             base += ', digest="%s"' % entdig
         if qop:
-            base += ', qop=auth, nc=%s, cnonce="%s"' % (ncvalue, cnonce)
+            base += ', qop="auth", nc=%s, cnonce="%s"' % (ncvalue, cnonce)
 
         return 'Digest %s' % (base)