diff --git a/app/models/user.rb b/app/models/user.rb
index 69bcfe2a8..993729a2f 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -86,7 +86,7 @@ class User
       group_ids = options.delete(:to)
     end
 
-    group_ids = [group_ids] if group_ids.is_a? BSON::ObjectId
+    group_ids = [group_ids.to_s] if group_ids.is_a? BSON::ObjectId
     raise ArgumentError.new("You must post to someone.") if group_ids.nil? || group_ids.empty?
 
     post = build_post(class_name, options)