diff --git a/requirements.txt b/requirements.txt
index 391cc9e9b..5acda7f80 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -47,7 +47,7 @@ tqdm>=3.7.0
 Trac>=1.0.10,<1.2
 Unidecode>=0.4.18
 #wsgiref>=0.1.2
-xml2rfc>=2.5.
+xml2rfc>=2.5.0,!=2.6.0
 xym>=0.1.2,!=0.3
 #zxcvbn-python>=4.4.14		# Not needed until we do back-end password entropy validation