 <p>For me, this ERROR was the result of utilizing NSURLProtocol and the "setProperty" method.</p> <p>[NSURLProtocol setProperty:FOO forKey:BAR inRequest:newRequest];</p> <p>I am not exactly certain of what is going on here, but from my experience I had two NSURLProtocol implementations. One needed property (FOO) and the other didn't. </p> <p>Core Code sets (FOO) NSURLProtocol #1 gets (FOO) and handles the COMM itself --- no issue</p> <p>NSURLProtocol #2 doesn't access (FOO) and hands of the COMM to a SESSION -- shows the error above</p> <p>Commenting out the Core Code that sets the (FOO) property solves the problem for NSURLProtocol #2. Obviously if you are using 3rd-party code, this would be more difficult to manage.</p> <p>BTW, in my case this "ERROR" didn't break the follow of logic and operated more as a warning.</p>