diff --git a/services/pull/compare.go b/services/pull/compare.go
index 00a74f2972..ce9210b22d 100644
--- a/services/pull/compare.go
+++ b/services/pull/compare.go
@@ -57,6 +57,12 @@ func GetCompareInfo(ctx context.Context, baseRepo, headRepo *repo_model.Reposito
 		compareInfo.MergeBase = compareInfo.BaseCommitID
 	}
 
+	if baseRepo.ID != headRepo.ID {
+		if err := gitrepo.FetchRemoteCommit(ctx, headRepo, baseRepo, compareInfo.BaseCommitID); err != nil {
+			return nil, fmt.Errorf("FetchRemoteCommit: %w", err)
+		}
+	}
+
 	// Count number of changed files.
 	// This probably should be removed as we need to use shortstat elsewhere
 	// Now there is git diff --shortstat but this appears to be slower than simply iterating with --nameonly