Wouldn't this work instead of a nil default?

error extension

extension NSError{
	//...
	class func
		myParseError(description:String?="Parse failed.") -> NSError {
			let info = [NSLocalizedDescriptionKey: description]
			
			return self(
				domain:myDomain,
				code:MyErrorCodes.ParseError.rawValue,
				userInfo:info)
	}
}

// Original from http://www.figure.ink/blog/2015/3/21/clean-optional-parameters