diff --git a/web/Dockerfile b/web/Dockerfile
index 4f943dc..5c1137c 100644
--- a/web/Dockerfile
+++ b/web/Dockerfile
@@ -31,6 +31,7 @@ RUN set -ex \
 		django-auth-ldap \
 		python-memcached \
 		diskcache \
+		django-utils-six \
 	&& apk del .build-deps \
 	&& addgroup -S mailman \
 	&& adduser -S -G mailman mailman \
diff --git a/web/Dockerfile.dev b/web/Dockerfile.dev
index d3e7753..9005f17 100644
--- a/web/Dockerfile.dev
+++ b/web/Dockerfile.dev
@@ -35,6 +35,7 @@ RUN set -ex \
         django-auth-ldap \
 		python-memcached \
         diskcache \
+        django-utils-six \
     && python3 -m pip install -U 'Django<3.1' \
     && python3 -m pip install -U \
        git+https://gitlab.com/mailman/django-mailman3@${DJ_MM3_REF} \