diff --git a/.github/workflows/docker-build.yml b/.github/workflows/docker-build.yml
index 2cda7475..9029ac24 100644
--- a/.github/workflows/docker-build.yml
+++ b/.github/workflows/docker-build.yml
@@ -42,7 +42,7 @@ env:
   TRIGGER_HEAD_SHA: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.head_sha || github.sha }}
   TRIGGER_REF: ${{ github.event_name == 'workflow_run' && format('refs/heads/{0}', github.event.workflow_run.head_branch) || github.ref }}
   TRIGGER_HEAD_REF: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.head_branch || github.head_ref }}
-  TRIGGER_PR_NUMBER: ${{ github.event_name == 'workflow_run' && join(github.event.workflow_run.pull_requests.*.number, '') || github.event.pull_request.number }}
+  TRIGGER_PR_NUMBER: ${{ github.event_name == 'workflow_run' && join(github.event.workflow_run.pull_requests.*.number, '') || format('{0}', github.event.pull_request.number) }}
   TRIGGER_ACTOR: ${{ github.event_name == 'workflow_run' && github.event.workflow_run.actor.login || github.actor }}
 
 jobs: