diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index e658e5a..a8e0b83 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.9, 3.10]
+        python-version: ['3.9', '3.10']
     env:
       BUFR_ORIGINATING_CENTRE: 123
       BUFR_ORIGINATING_SUBCENTRE: 123