diff --git a/.github/workflows/modgc.yml b/.github/workflows/modgc.yml
index 83fd819..b269d41 100644
--- a/.github/workflows/modgc.yml
+++ b/.github/workflows/modgc.yml
@@ -91,6 +91,7 @@ jobs:
         working-directory: ./res/ruby
         env:
           RUBY_TESTOPTS: '-q --tty=no'
+          RUBY_GC_LIBRARY: mmtk
           TEST_BUNDLED_GEMS_ALLOW_FAILURES: 'typeprof'
           PRECHECK_BUNDLED_GEMS: 'no'