diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 624c64a4..1779bfd7 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -171,7 +171,7 @@ jobs:
           -e TMP=/scratch
           -e SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1
           debos -v
-          ${{matrix.backend == 'nofakemachine' && '--disable-fakemachine' || join(['--fakemachine-backend=',matrix.backend]) }}
+          ${{matrix.backend == 'nofakemachine' && '--disable-fakemachine' || format('{0}{1}', '--fakemachine-backend=',matrix.backend) }}
           ${{matrix.test.variables}}
           ${{matrix.test.case}}/test.yaml