diff --git a/.github/workflows/modgc.yml b/.github/workflows/modgc.yml
index 83fd819..60e4b0f 100644
--- a/.github/workflows/modgc.yml
+++ b/.github/workflows/modgc.yml
@@ -54,6 +54,7 @@ jobs:
             echo 'SYNTAX_SUGGEST_TIMEOUT=60' >> $GITHUB_ENV
           fi
           echo 'MMTK_PLAN=${{ matrix.gc.mmtk_plan }}' >> $GITHUB_ENV
+          echo 'RUBY_GC_LIBRARY=mmtk' >> $GITHUB_ENV
 
         if: ${{ matrix.gc.name == 'mmtk' }}
       - uses: ruby/setup-ruby@a6e6f86333f0a2523ece813039b8b4be04560854 # v1.190.0