diff --git a/diff-sha.sh b/diff-sha.sh
index 9435ea1a..154df435 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -203,7 +203,7 @@ else
       PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
       
       if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA"..."$CURRENT_SHA" 1>/dev/null 2>&1; then
-        PREVIOUS_SHA=$(git merge-base origin/"$TARGET_BRANCH" HEAD 2>&1) && exit_status=$? || exit_status=$?
+        PREVIOUS_SHA=$(git rev-parse origin/"$TARGET_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
       fi
     fi
 
@@ -230,6 +230,8 @@ else
         # shellcheck disable=SC2086
         git fetch $EXTRA_ARGS -u --progress --deepen="$depth" origin +"$GITHUB_REF":refs/remotes/origin/"$CURRENT_BRANCH"
 
+        PREVIOUS_SHA=$(git merge-base origin/"$TARGET_BRANCH" "$CURRENT_SHA" 2>&1)
+
         if [[ $depth -gt $max_depth ]]; then
           echo "::error::Unable to locate a common ancestor between $TARGET_BRANCH and $CURRENT_SHA"
           exit 1