diff --git a/mspec/runner/guards/platform.rb b/mspec/runner/guards/platform.rb
index 4ea5128..515f5bc 100644
--- a/mspec/runner/guards/platform.rb
+++ b/mspec/runner/guards/platform.rb
@@ -39,13 +39,15 @@ end
 class Object
   def platform_is(*args)
     g = PlatformGuard.new(*args)
-    yield if g.yield?
+    result = g.yield? ? yield : nil
     g.unregister
+    result
   end
   
   def platform_is_not(*args)
     g = PlatformGuard.new(*args)
-    yield if g.yield? true
+    result = g.yield?(true) ? yield : nil
     g.unregister
+    result
   end
 end