A simple isNull for string functionality with Active Patterns

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open System

let (|Empty|Null|String|) (s:string) =
    if s = null then
        Null
    elif s.Trim() |> String.IsNullOrEmpty then
        Empty
    else
        String (s.Trim())

let public IsNull (rep: string) (s:string) = 
    match s with
    | Empty | Null -> rep.ToString()
    | String _     -> s

If someone knows why the underlined ToString() is needed I would appreciate it