diff --git a/.github/workflows/wiki-guard.yml b/.github/workflows/wiki-guard.yml
index 709fb8ea..dbe3dd8d 100644
--- a/.github/workflows/wiki-guard.yml
+++ b/.github/workflows/wiki-guard.yml
@@ -37,6 +37,9 @@ concurrency:
 
 jobs:
   guard:
+    # Forks have no <fork>.wiki repository, so the checkout below would
+    # fail there -- run only in the upstream repository.
+    if: github.repository == 'acmesh-official/acme.sh'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout wiki repository