diff --git a/lib/motion/spec.rb b/lib/motion/spec.rb
index e230f247..6aba2ec5 100644
--- a/lib/motion/spec.rb
+++ b/lib/motion/spec.rb
@@ -511,7 +511,7 @@ module Bacon
     if defined?(NSObject)
       current_context.performSelector("run", withObject:nil, afterDelay:0)
     else
-      @main_activity = arg
+      @main_activity ||= arg
       current_context.run
     end
   end