diff --git a/.github/workflows/diff-pr.yml b/.github/workflows/diff-pr.yml
index 17b0abf..e363fc1 100644
--- a/.github/workflows/diff-pr.yml
+++ b/.github/workflows/diff-pr.yml
@@ -20,7 +20,9 @@ jobs:
           fetch-depth: 0
 
       - name: worktree base for diff
-        run: git worktree add "$RUNNER_TEMP/base" "${{ github.event.pull_request.base.sha }}"
+        run: |
+          git fetch origin ${{ github.event.pull_request.base.ref }}
+          git worktree add "$RUNNER_TEMP/base" "origin/${{ github.event.pull_request.base.ref }}"
 
       - name: build closures and render diff
         run: |