Ambiguous use of Function Error

Question

I use the following code,

@IBOutlet weak var hashtagableTextView: UITextView!

In viewDidload method:

let hashtagableDelegate = HashtagableTextInputDelegate(textView: hashtagableTextView, userDelegate: self)

hashtagableDelegate.didStartTypingHashtag = { partialHashtag in
    let searchableString = partialHashtag.replacingOccurrences(of: "#", with: "")
    hashtagableDelegate.showSuggestedHashtags(hashtagDatabase.filter({ $0.lowercased().contains(searchableString.lowercased()) }))
}

hashtagableTextView.delegate = hashtagableDelegate

I'm getting error like

ambiguous use of contains

How can i fix?