Attributed string loses attributes inside function

Question

I have an attributed string I create like so:

let myMutableString = NSMutableAttributedString(
            string:  "Father order number:",
            attributes: [NSFontAttributeName:UIFont(
                name: "Helvetica Neue",
                size: 18.0)!])

        myMutableString.addAttribute(NSForegroundColorAttributeName, value: UIColor.darkGray, range: NSRange(location: 0, length: myMutableString.length-1))

        label1.attributedText = myMutableString

This works fine, and the attributes show up on the simulator. However, when I try to put the attributed string into a function like so, the attributes don't show up when I make the function call.

function call:

let myMutableString:NSMutableAttributedString! = getDarkGrayHelveticaAttributedString(str: "Father order number:")

function use:

label1.attributedText = myMutableString

function:

func getDarkGrayHelveticaAttributedString(str: String) -> NSMutableAttributedString {
    var myMutableString = NSMutableAttributedString(
        string:  str,
        attributes: [NSFontAttributeName:UIFont(
            name: "Helvetica Neue",
            size: 18.0)!])

    return myMutableString
}