diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index b12a878327d27ca50920333a3593e41ef169dade..1245cc16d6ada0659d731bef5884eca735e2d946 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -332,14 +332,14 @@ class MergeRequest < ActiveRecord::Base
   #
   # see "git diff"
   def to_diff
-    target_project.repository.diff_text(diff_base_commit, source_sha)
+    target_project.repository.diff_text(diff_base_commit.sha, source_sha)
   end
 
   # Returns the commit as a series of email patches.
   #
   # see "git format-patch"
   def to_patch
-    target_project.repository.format_patch(diff_base_commit, source_sha)
+    target_project.repository.format_patch(diff_base_commit.sha, source_sha)
   end
 
   def hook_attrs