diff --git a/diff-sha.sh b/diff-sha.sh
index 0660f86d..2f1ff268 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -93,7 +93,7 @@ function deepenShallowCloneToFindCommitPullRequest() {
   local depth=20
   local max_depth=$INPUT_MAX_FETCH_DEPTH
 
-  while [ -z "$(git merge-base "$target_branch" "$current_branch")" ]; do
+  while ! git diff "$target_branch"..."$current_branch"; do
     echo "::debug::Unable to find merge-base in shallow clone. Increasing depth to $((depth * 2))..."
 
     depth=$((depth * 2))