Objective C - Change all attributes in NSAttributedString?

[attributedString enumerateAttributesInRange:range options:NSAttributedStringEnumerationReverse usingBlock:
     ^(NSDictionary *attributes, NSRange range, BOOL *stop) {

         NSMutableDictionary *mutableAttributes = [NSMutableDictionary dictionaryWithDictionary:attributes];
         [mutableAttributes setObject:[NSNumber numberWithInt:1] forKey:@"NSUnderline"];
         attributes = mutableAttributes;

     }];

I am trying to loop through all attributed and add NSUnderline to them. when debugging it seems like NSUnderline is added to the dictionary, but when i loop for the second time they are removed. Am I doing anything wrong while updating NSDictionaries?