diff --git a/CHANGELOG.md b/CHANGELOG.md
index 25ecc53..7490cfd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,8 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix forum starting issue: "NoMethodError: undefined method `encode' for nil:NilClass".
+
 ## v12.1.0 (2021-09-17)
 
 - [Improvement] Make it easier to run edx-platform unit tests.
diff --git a/tutor/templates/build/forum/Dockerfile b/tutor/templates/build/forum/Dockerfile
index 6d48eaf..2bb7c23 100644
--- a/tutor/templates/build/forum/Dockerfile
+++ b/tutor/templates/build/forum/Dockerfile
@@ -45,6 +45,7 @@ ENV NEW_RELIC_ENABLE false
 ENV API_KEY forumapikey
 ENV SEARCH_SERVER "http://elasticsearch:9200"
 ENV MONGODB_AUTH ""
+ENV MONGOID_AUTH_MECH ":scram"
 ENV MONGODB_HOST "mongodb"
 ENV MONGODB_PORT "27017"
 ENV MONGODB_DATABASE "cs_comments_service"