Random string split into characters

I have a procedure, that generates a random string without any delimiter. I store return value of this as a string, but I would like to split this text into characters and after that examine each characters. I try to use above code to split string into characters, but it gives Type mismatch error.

Sub gen()
    Dim s As String
    s = textgen(4000, 5)
    Dim buff() As String
    ReDim buff(Len(s) - 1)

    For i = 1 To Len(s)
        buff(i - 1) = Mid$(s, i, 1)
    Next

    MsgBox (buff)  ' type mismatch
End Sub