get an error in xcode when i switch haar xml files?

Hi for some reason i am using the face tracking example and haarFinder.setup(“haarXML/haarcascade_frontalface_default.xml”); which works great!
but when i change the xml file in this line to another one in the same folder like “mouth.xml”
i get an error:

GDB: Error could not locate source file: cxerror.cpp(line: 359)

and in the debugger this is the line that is highlighted:

cvHaarDetectObjects(hack.getCvImage(), cascade, storage, scaleHaar, 2, CV_HAAR_DO_CANNY_PRUNING);

does anyone have any clue why this error is happening i thought you could just replace the xml
in the haar finder setup and it will detect different things? is there another step i have to follow?
thanks!