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

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

However when the substring is an optional as in the following,

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

I am getting the error

"No exact matches in call to initializer "

What could be causing this error?

Thanks for any suggestions.