diff --git a/ractor.rb b/ractor.rb
index caeca88644..b22cb68939 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -274,7 +274,7 @@ class Ractor
     b = block # TODO: builtin bug
     raise ArgumentError, "must be called with a block" unless block
     if __builtin_cexpr!("RBOOL(ruby_single_main_ractor)")
-      warn("Ractor is experimental, and the behavior may change in future versions of Ruby! " \
+      Kernel.warn("Ractor is experimental, and the behavior may change in future versions of Ruby! " \
            "Also there are many implementation issues.", uplevel: 0, category: :experimental)
     end
     loc = caller_locations(1, 1).first