Bad Lexical Cast? ? ?

daytrans = 0.01;
 coroutine.wrap(function()
    for i = 14,5,-daytrans do
     Lighting.TimeOfDay = i; 
     wait();
    end
   end)();

This gives me the error: 17:38:21.702 - bad lexical cast: source type value could not be interpreted as target

Why? Any ideas?