diff --git a/diff-sha.sh b/diff-sha.sh
index 7c51a5ed..53649057 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -224,7 +224,11 @@ else
     if [[ "$INPUT_SINCE_LAST_REMOTE_COMMIT" == "true" ]]; then
       PREVIOUS_SHA=$GITHUB_EVENT_BEFORE
     else
-      PREVIOUS_SHA=$(git merge-base --fork-point "$TARGET_BRANCH" "$CURRENT_SHA") && exit_status=$? || exit_status=$?
+      PREVIOUS_SHA=${COMMON_ANCESTOR:-}
+      
+      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 "$PREVIOUS_SHA$DIFF$CURRENT_SHA" 1>/dev/null 2>&1; then
         PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA