Swift struct extension add initializer

I'm trying to add an initializer to Range.

import Foundation

extension Range {
    init(_ range: NSRange, in string: String) {
        let lower = string.index(string.startIndex, offsetBy: range.location)
        let upper = string.index(string.startIndex, offsetBy: NSMaxRange(range))
        self.init(uncheckedBounds: (lower: lower, upper: upper))
    }
}

But, the last line has a Swift compiler error.

Cannot convert value of type '(lower: String.Index, upper: String.Index)' (aka '(lower: String.CharacterView.Index, upper: String.CharacterView.Index)') to expected argument type '(lower: _, upper: _)'

How do I get it to compile?