Dynamic search function for UITableView in Swift

I'm trying to create a search function for my table view, but when I type in the second character in my text field, the function crashes. I can type in the first character, delete it, and type in a new one and it doesn't crash.

My error says:

fatal error: can not increment endIndex

and my code looks like this:

func textFieldDidChange(textField: UITextField){
    self.loadSearchData(textField.text)
}

func loadSearchData(searchString:String){

    var arrayOfSearches: [String] = []
    let index = count(searchString)

    for company in self.companies{
        let searchIndex = advance(company.startIndex, index)
        if searchString == company.substringToIndex(searchIndex){
        arrayOfSearches.append(company)
        }
    }
    self.companiesToDisplay = arrayOfSearches
    self.companiesTV.reloadData()
}

where my companiesToDisplay is the array being displayed by my table view.

I know the fault is in my let searchIndex = advance(company.startIndex, index) line but I don't know why it would create an error.

Any suggestions on how to solve this would be appreciated.