diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7299e80..6b397c6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,7 +9,7 @@ on:
 jobs:
   mix_test:
     name: mix test (Elixir ${{matrix.elixir}} | OTP ${{matrix.otp}})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: false
       matrix: