diff --git a/scripts/lib/custom-docker-build b/scripts/lib/custom-docker-build
index 1c8292d..9162ba8 100755
--- a/scripts/lib/custom-docker-build
+++ b/scripts/lib/custom-docker-build
@@ -334,7 +334,7 @@ function parse_arguments() {
             version="${!tool}"
             case "$tool" in
                 ARCH) CUSTOM_DOCKER_ARCH=$version ;;
-                OS) CUSTOM_BASE_IMAGE=get_base_image_reference $version ;;
+                OS) CUSTOM_BASE_IMAGE=$(get_base_image_reference $version) ;;
                 RUBY) print_ruby_args $version ;;
                 BUNDLER) print_bundler_args $version ;;
                 RUBYGEMS) print_rubygems_args $version ;;