diff --git a/.github/workflows/R-CMD-check.yaml b/.github/workflows/R-CMD-check.yaml
index 9624132..75259d3 100644
--- a/.github/workflows/R-CMD-check.yaml
+++ b/.github/workflows/R-CMD-check.yaml
@@ -22,9 +22,9 @@ jobs:
           - {os: windows-latest, r: '4.1'}
           - {os: windows-latest, r: 'release'}
           - {os: windows-latest, r: 'devel'}
-          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
-          - {os: ubuntu-latest,   r: 'release'}
-          - {os: ubuntu-latest,    r: 'oldrel-4'}
+          - {os: ubuntu-20.04,   r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-20.04,   r: 'release'}
+          - {os: ubuntu-20.04,   r: 'oldrel-4'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}