This code here is working for me
NSString *s = @"avant";
s = [s stringByAppendingString:@" - après"];
NSLog(@"%@", s);
2012-01-13 11:48:59.442 tabbar[604:207] avant – après
So my guess is that your you is a bad pointer that is not nil and not the NSString you think it have.
Have you try an NSLog on that value before the call?