diff --git a/diff-sha.sh b/diff-sha.sh
index 154df435..43346aa5 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -200,8 +200,12 @@ else
         PREVIOUS_SHA=$(git rev-parse origin/"$CURRENT_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
       fi
     else
-      PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
-      
+      PREVIOUS_SHA=$(git merge-base "$TARGET_BRANCH" "$CURRENT_SHA" 2>&1) && exit_status=$? || exit_status=$?
+
+      if [[ -z "$PREVIOUS_SHA" ]]; then
+        PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
+      fi
+
       if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA"..."$CURRENT_SHA" 1>/dev/null 2>&1; then
         PREVIOUS_SHA=$(git rev-parse origin/"$TARGET_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
       fi