Apple’s documentation and many articles suggest you can convert a substring (string.subsequence) to a string just by calling String(substring)

let str = "Hello world"
let hello = str.prefix(5) //create substr
//CREATE STRING FROM SUBSTRING
let hellostr = String(hello) //gives error 

However I am getting the error

"No exact matches in call to initializer "

What could be causing this error?

Thanks for any suggestions.