iOS - Issues adding kCGImagePropertyExifUserComment to image

I'm having problems writing a kCGImagePropertyExifUserComment to my image. My code (overall) works, as it writes GPS coordinates without a hassle. However, writing some additional info in the UserComment field is non-working (and I'm not sure why). Code is:

CFDictionarySetValue(mutable, kCGImagePropertyExifUserComment, [NSString stringWithFormat:@"%0开发者_StackOverflow社区.0f,%0.0f,%0.0f",yaw,pitch,roll]);

Anyone have an idea on this?

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If anyone else is wondering how to do this, I've finally figured it out:

NSDictionary *etcDict=[NSDictionary dictionaryWithObjectsAndKeys:
                               [NSString stringWithFormat:@"%0.0f,%0.0f,%0.0f",yaw,pitch,roll],kCGImagePropertyExifUserComment,
                               nil];
CFDictionarySetValue(mutable, kCGImagePropertyExifDictionary, etcDict);

Strange, but it works just fine!