Hi, I'm sure it's something really simple but the following code in my project: NSMutableString *temp = [[NSMutableString alloc] initWithString:[firstNumber stringValue]]; NSArray *splitInput = [[NSArray alloc]initWithArray:[temp componentsSeparatedByString"x"]]; NSMutableString *f = ( NSMutableString * )[splitInput objectAtIndex:1]; [f insertString: ( NSMutableString * ) @"00000000" atIndex:0]; Where 'firstNumber' is the outlet from a standard text field in the main window. It produces the error: Attempt to mutate immutable object with insertString:atIndex: My code's a very early stage timecode calculator, so the user can enter '4x23' and the '23' needs to ultimately be padded with zeroes. I realise there's probably a better way by using 'NSNumberFormatter', but now I've a flea in my ear and I'd just like to know what I've done wrong. Many thanks! Todd.