Why is this semantically equivalent (as far as I can see) vbscript code fails

I want to create a FormatString function for VBScript that works the same as String.Format in .Net.

I found I can use the System.Text.StringBuilder object in VBScript and tested the following code that works

Option Explicit

Dim sbText 'As System.Text.StringBuilder
Set sbText = CreateObject("System.Text.StringBuilder")

Call sbText.AppendFormat_5( _
                Nothing, _
                "My name is {0} and the current date time is '{1:dd MMMM yyyy HH:mm:ss}'", _
                Array("Robert", Now))


Call MsgBox(sbText.ToString())
      

        
        
        
      

    

Then I went to put it in a function and it doesn't show below

Option Explicit

Function FormatString(ByVal sText, ByVal Arguments) 'As String

    Dim sbText 'As System.Text.StringBuilder

    'Test the input variables
    If Not TypeName(sText) = "String" Then _
        Err.Raise 5 'vbErrInvalidProcCallOrArg

    If Not IsArray(Arguments) Then _
        Err.Raise 5 'vbErrInvalidProcCallOrArg

    Set sbText = CreateObject("System.Text.StringBuilder")
    Call sbText.AppendFormat_5(Nothing, sText, Arguments)

    FormatString = sbText.ToString()

End Function

Call MsgBox(FormatString( _
            "My name is {0} and the current date time is '{1:dd MMMM yyyy HH:mm:ss}'", _
            Array("Robert", Now)))
      

        
        
        
      

    

Error Call sbText.AppendFormat_5(Nothing, sText, Arguments)

with error "Invalid procedure call or argument:" sbText.AppendFormat_5 ".

So I don't understand why outside the function I can pass the following types in order:

Nothing
String
Contrary

and they work, but inside a function that it is not.

Can anyone please help?