diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index d743637eae..994c92ea0a 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -9,7 +9,7 @@ on:
 name: phpunit
 jobs:
   phpunit:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         php-versions: ['7.3', '7.4']