diff --git a/action.yml b/action.yml
index 43e1463a..31330112 100644
--- a/action.yml
+++ b/action.yml
@@ -173,8 +173,9 @@ runs:
         GITHUB_BASE_REF: ${{ github.base_ref }}
         GITHUB_HEAD_REF: ${{ github.head_ref }}
         GITHUB_WORKSPACE: ${{ github.workspace }}
-        GITHUB_HEAD_REPO_FORK: ${{ github.event.pull_request.head.repo.fork }}
+        GITHUB_EVENT_HEAD_REPO_FORK: ${{ github.event.pull_request.head.repo.fork }}
         GITHUB_EVENT_PULL_REQUEST_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+        GITHUB_EVENT_PULL_REQUEST_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
         GITHUB_EVENT_BEFORE: ${{ github.event.before }}
         GITHUB_EVENT_FORCED: ${{ github.event.forced }}
         GITHUB_REFNAME: ${{ github.ref_name }}
diff --git a/diff-sha.sh b/diff-sha.sh
index 01b41458..b6c5e9a5 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -13,7 +13,7 @@ if [[ "$GITHUB_REF" == "refs/tags/"* ]]; then
   EXTRA_ARGS=""
 fi
 
-if [[ "$GITHUB_HEAD_REPO_FORK" == "true" ]]; then
+if [[ "$GITHUB_EVENT_HEAD_REPO_FORK" == "true" ]]; then
   MERGE_BASE_EXRTRA_ARGS="--fork-point"
 fi