NSMutableAttributedStrings - objectAtIndex:effectiveRange:: Out of bounds

I'm trying to add some fancy text to a label, but I've run into some problems with the NSMutableAttributedString class. I was trying to achieve four this: 1. Change font, 2. Underline range, 3. Change range color, 4. Superscript range.

This code:

- (void)applicationDidFinishLaunching:(NSNotification*)aNotification
{
    NSMutableAttributedString* display = [[NSMutableAttributedString alloc]
                                          initWithString:@"Hello world!"];
    NSUInteger length = [[display string]length] - 1;

    NSRange wholeRange = NSMakeRange(0, length);
    NSRange helloRange = NSMakeRange(0, 4);
    NSRange worldRange = NSMakeRange(6, length);

    NSFont* monoSpaced = [NSFont fontWithName:@"Menlo" 
                                         size:22.0];

    [display addAttribute:NSFontAttributeName
                    value:monoSpaced
                    range:wholeRange];

    [display addAttribute:NSUnderlineStyleAttributeName 
                    value:[NSNumber numberWithInt:1] 
                    range:helloRange];

    [display addAttribute:NSForegroundColorAttributeName 
                    value:[NSColor greenColor]
                    range:helloRange];

    [display addAttribute:NSSuperscriptAttributeName 
                    value:[NSNumber numberWithInt:1] 
                    range:worldRange];

    //@synthesize textLabel; is in this file.
    [textLabel setAttributedStringValue:display];
}

Gives me this error:

NSMutableRLEArray objectAtIndex:effectiveRange:: Out of bounds

Also, I tried messing around with the ranges, but became even more confused when I tried NSRange worldRange = NSMakeRange(4, 5);. I don't understand why that produces this: Hell^o wor^ld!, where the letters inside the ^s are superscripts.

NSRange worldRange = NSMakeRange(6, 6); produces the desired effect, hello ^world!^.

What the label looks like: