Fix Error 500 when comparing non-existing branchesfix-issue-2593

Closes #2593

1 files changed, 4 insertions, 1 deletions

diff --git a/app/services/compare_service.rb b/app/services/compare_service.rb
index 70f642baaaa..bfe6a3dc4be 100644
--- a/app/services/compare_service.rb
+++ b/app/services/compare_service.rb
@@ -4,7 +4,10 @@ require 'securerandom'
 # and return Gitlab::CompareResult object that responds to commits and diffs
 class CompareService
   def execute(source_project, source_branch, target_project, target_branch)
-    source_sha = source_project.commit(source_branch).sha
+    source_commit = source_project.commit(source_branch)
+    return unless source_commit
+
+    source_sha = source_commit.sha
 
     # If compare with other project we need to fetch ref first
     unless target_project == source_project