diff --git a/js/src/forum/components/CommentPost.js b/js/src/forum/components/CommentPost.js
index e3c90cbc4..1c7020322 100644
--- a/js/src/forum/components/CommentPost.js
+++ b/js/src/forum/components/CommentPost.js
@@ -80,7 +80,7 @@ export default class CommentPost extends Post {
     const post = this.props.post;
     const attrs = super.attrs();
 
-    attrs.className += ' '+classList({
+    attrs.className = (attrs.className || '') + ' ' + classList({
       'CommentPost': true,
       'Post--hidden': post.isHidden(),
       'Post--edited': post.isEdited(),
diff --git a/js/src/forum/components/EventPost.js b/js/src/forum/components/EventPost.js
index f4645275c..d059e7dc0 100644
--- a/js/src/forum/components/EventPost.js
+++ b/js/src/forum/components/EventPost.js
@@ -18,7 +18,7 @@ export default class EventPost extends Post {
   attrs() {
     const attrs = super.attrs();
 
-    attrs.className += ' EventPost ' + ucfirst(this.props.post.contentType()) + 'Post';
+    attrs.className = (attrs.className || '') + ' EventPost ' + ucfirst(this.props.post.contentType()) + 'Post';
 
     return attrs;
   }