diff --git a/gc/default.c b/gc/default.c
index 5b7e427e3e..1ecffe9b1d 100644
--- a/gc/default.c
+++ b/gc/default.c
@@ -8035,7 +8035,7 @@ gc_config_set_key(st_data_t key, st_data_t value, st_data_t data)
     rb_objspace_t *objspace = (rb_objspace_t *)data;
     if (rb_sym2id(key) == rb_intern("rgengc_allow_full_mark")) {
         gc_rest(objspace);
-        gc_config_full_mark_set(RBOOL(value));
+        gc_config_full_mark_set(RTEST(value));
     }
     return ST_CONTINUE;
 }
diff --git a/test/ruby/test_gc.rb b/test/ruby/test_gc.rb
index f47f1e282a..a05da28f7e 100644
--- a/test/ruby/test_gc.rb
+++ b/test/ruby/test_gc.rb
@@ -74,8 +74,10 @@ class TestGc < Test::Unit::TestCase
 
     new_value = GC.config(rgengc_allow_full_mark: false)[:rgengc_allow_full_mark]
     assert_false(new_value)
+    new_value = GC.config(rgengc_allow_full_mark: nil)[:rgengc_allow_full_mark]
+    assert_false(new_value)
   ensure
-    GC.config(rgengc_allow_full_mark: true)
+    GC.config(rgengc_allow_full_mark: old_value)
     GC.start
   end