You need to post the code for your MockPhotoSource method initWithType: etc.
We've only been guessing what its parameters are, since you've never shown them.
Based on the error message, I could guess what the problem is, but it's a lot more accurate if we have your real code to look at, rather than imagining what code you might have written, and trying to debug that imaginary code.
You should also get in the habit of finding results by yourself. If I enter google search terms xcode missing sentinel in function call, a number of results appear in which other programmers have seen this message and discovered why it occurs. All I did was enter the exact text of the error message, and precede it with the word xcode to tell google that I only wanted Xcode-related search results. This is one of the simplest of strategies for finding what the cause of an error is, yet it works surprisingly well most of the time.
