diff --git a/src/Core/Post/DiscussionRenamedPost.php b/src/Core/Post/DiscussionRenamedPost.php
index 29cec3264..fd34e7402 100755
--- a/src/Core/Post/DiscussionRenamedPost.php
+++ b/src/Core/Post/DiscussionRenamedPost.php
@@ -28,7 +28,7 @@ class DiscussionRenamedPost extends AbstractEventPost implements MergeableInterf
     /**
      * {@inheritdoc}
      */
-    public function saveAfter(Post $previous)
+    public function saveAfter(Post $previous = null)
     {
         // If the previous post is another 'discussion renamed' post, and it's
         // by the same user, then we can merge this post into it. If we find
diff --git a/src/Core/Post/MergeableInterface.php b/src/Core/Post/MergeableInterface.php
index 4feca3e66..0e84d3a23 100644
--- a/src/Core/Post/MergeableInterface.php
+++ b/src/Core/Post/MergeableInterface.php
@@ -25,10 +25,10 @@ interface MergeableInterface
      * Save the model, given that it is going to appear immediately after the
      * passed model.
      *
-     * @param Post $previous
+     * @param Post|null $previous
      * @return Post The model resulting after the merge. If the merge is
      *     unsuccessful, this should be the current model instance. Otherwise,
      *     it should be the model that was merged into.
      */
-    public function saveAfter(Post $previous);
+    public function saveAfter(Post $previous = null);
 }