diff --git a/app/models/repository.rb b/app/models/repository.rb
index 7f0047a002e0dc8a3101cbdbb4c887d719177823..a5787a052778fda2e22bc2e103c279762fca0b63 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -611,6 +611,8 @@ class Repository
   end
 
   def merge_base(first_commit_id, second_commit_id)
+    first_commit_id = commit(first_commit_id).try(:id) || first_commit_id
+    second_commit_id = commit(second_commit_id).try(:id) || second_commit_id
     rugged.merge_base(first_commit_id, second_commit_id)
   rescue Rugged::ReferenceError
     nil