Crash Log When Setting Priority With Cartography

gistfile1.txt

2014-10-23 09:41:04.083 Tester[2596:54628] *** Assertion failure in -[NSLayoutConstraint setPriority:], /SourceCache/Foundation_Sim/Foundation-1141.1/Layout.subproj/NSLayoutConstraint.m:174
2014-10-23 09:41:04.087 Tester[2596:54628] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Mutating a priority from required to not on an installed constraint (or vice-versa) is not supported.  You passed priority 750 and the existing priority was 1000.'
*** First throw call stack:
(
	0   CoreFoundation                      0x0000000108894f35 __exceptionPreprocess + 165
	1   libobjc.A.dylib                     0x000000010a3d8bb7 objc_exception_throw + 45
	2   CoreFoundation                      0x0000000108894d9a +[NSException raise:format:arguments:] + 106
	3   Foundation                          0x0000000108d2c5df -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 195
	4   Foundation                          0x0000000108ca8b3c -[NSLayoutConstraint setPriority:] + 331
	5   Tester                              0x00000001086865a3 _TFC6Tester14ViewController11viewDidLoadfS0_FT_T_ + 1203
	6   Tester                              0x00000001086865f2 _TToFC6Tester14ViewController11viewDidLoadfS0_FT_T_ + 34
	7   UIKit                               0x0000000109245a90 -[UIViewController loadViewIfRequired] + 738
	8   UIKit                               0x0000000109245c8e -[UIViewController view] + 27
	9   UIKit                               0x0000000109164ca9 -[UIWindow addRootViewControllerViewIfPossible] + 58
	10  UIKit                               0x0000000109165041 -[UIWindow _setHidden:forced:] + 247
	11  UIKit                               0x000000010917172c -[UIWindow makeKeyAndVisible] + 42
	12  UIKit                               0x000000010911c061 -[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 2628
	13  UIKit                               0x000000010911ed2c -[UIApplication _runWithMainScene:transitionContext:completion:] + 1350
	14  UIKit                               0x000000010911dbf2 -[UIApplication workspaceDidEndTransaction:] + 179
	15  FrontBoardServices                  0x000000010bf652a3 __31-[FBSSerialQueue performAsync:]_block_invoke + 16
	16  CoreFoundation                      0x00000001087ca53c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
	17  CoreFoundation                      0x00000001087c0285 __CFRunLoopDoBlocks + 341
	18  CoreFoundation                      0x00000001087bfa43 __CFRunLoopRun + 851
	19  CoreFoundation                      0x00000001087bf486 CFRunLoopRunSpecific + 470
	20  UIKit                               0x000000010911d669 -[UIApplication _run] + 413
	21  UIKit                               0x0000000109120420 UIApplicationMain + 1282
	22  Tester                              0x0000000108687e5e top_level_code + 78
	23  Tester                              0x0000000108687e9a main + 42
	24  libdyld.dylib                       0x000000010abb2145 start + 1
	25  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException