diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e7de2e1..d4cb5d5 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -29,5 +29,5 @@ Ruby 2.4:
 JRuby 9.1:
   image: "jruby:9.1-alpine"
   variables:
-    INSTALL_OPENJDK: true
+    INSTALL_OPENJDK: "true"
   <<: *defaults