Swift - Fatal Error on Performing Segue

My iOS app is crashing when I run the following line of code after the push of a standard UIButton: self.performSegueWithIdentifier("loginAttempt", sender: self) The segue identifier name is correct as I copied and pasted the name from the sidebar into the code. Upon remove the code the app stops crashing when the button is pressed. The error I get is: Thread 1: EXC_BREAKPOINT (code=1, subcode=0x1002544fc) And in the Output box: fatal error: unexpectedly found nil while unwrapping an Optional value