diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 514276f..6aaa96e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-20.04 ]
         ruby: [ 1.9, '2.0', 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, '3.0', 3.1, 3.2, 3.3, ruby-head, jruby, truffleruby ]
     runs-on: ${{ matrix.os }}
     steps: