diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8c3ba77..469f2e7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,7 +25,7 @@ jobs:
           if [[ $GITHUB_EVENT_NAME == push ]]; then
             upstream_branch="${GITHUB_REF#refs/heads/}"
           elif [[ $GITHUB_EVENT_NAME == pull_request ]]; then
-            upstream_branch="${GITHUB_HEAD_REF#refs/heads/}"
+            upstream_branch="${GITHUB_BASE_REF#refs/heads/}"
           else
             echo "unsupported event: $GITHUB_EVENT_NAME"
             exit 1
@@ -115,7 +115,7 @@ jobs:
           if [[ $GITHUB_EVENT_NAME == push ]]; then
             upstream_branch="${GITHUB_REF#refs/heads/}"
           elif [[ $GITHUB_EVENT_NAME == pull_request ]]; then
-            upstream_branch="${GITHUB_HEAD_REF#refs/heads/}"
+            upstream_branch="${GITHUB_BASE_REF#refs/heads/}"
           else
             echo "unsupported event: $GITHUB_EVENT_NAME"
             exit 1