diff --git a/spec/ruby/core/mutex/sleep_spec.rb b/spec/ruby/core/mutex/sleep_spec.rb
index 6638f5a5a2..9832e3125e 100644
--- a/spec/ruby/core/mutex/sleep_spec.rb
+++ b/spec/ruby/core/mutex/sleep_spec.rb
@@ -97,7 +97,7 @@ describe "Mutex#sleep" do
     m.lock
     times.each do |time|
       # just testing that sleep completes
-      m.sleep(time).should_not == nil
+      -> {m.sleep(time)}.should_not raise_error
     end
   end
 end