Need Advice

Hallo guys,I need advice.A guy approached me with an idea that needs a programmer (I am the programmer).After evaluation it's a 6 month to 12 month coding job without any payment. The other guy only contribution is the idea.Then after finishing the product this idea guy will use it for his business and give me 5% of the profit eg if the company makes 10 million and profit is 2.5 million then developer gets 0.05 X 2.5M = 125000. In addition to that if it's used as saas platform the idea guy owns 50% shares and me as developer 50%. And there is no clear answer how this product will be marketed. But this guy thinks this is a good idea and it will take off.Is this a good or bad deal?