Define function that can take null parameter

Question

I'm was recently trying to redefine Access's Nz(Value, [ValueIfNull]) function in Excel, because I find it pretty useful yet it's absent in Excel (as I found to my disappointment when moving a few useful functions over). Access's Nz function checks Value - if this value is null, it returns ValueIfNull (otherwise it returns Value). In Access it's useful for checking the value of input boxes (amongst several other things):

If Nz(myTextBox.Value, "") = "" Then
    MsgBox "You need to enter something!"
End If

Rolling my own Nz function didn't seem difficult:

Public Function Nz(value As Variant, Optional valueIfNull As Variant = "") As Variant
    If IsNull(value) Then
        Nz = valueIfNull
    Else
        Nz = value
    End If
End Function

But as soon as I try to call it with anything that's actually null, Excel complains about it on the calling line (Run-time error '91': Object variable or With block not set, which I understand to be roughly equivilant to a NullReferenceException in other languages), before even getting to the Nz function body. For example Nz(someObj.Value, "") will only work if someObj.Value isn't null (rendering the function entirely moot).

Am I missing some detail of VBA here? Coming from languages like VB.NET, it seems very confusing - I understand object references to be simply addresses to an actual object residing in memory, and so passing around the reference (not the object) shouldn't cause issue (until you try to actually do something with the non-existant object, of course). For eg:

Dim myObj As SomeObject
SomeMethod(myObj)  'the call itself is fine

Public Sub SomeMethod(SomeObject obj)
    myObj.DoSomething() 'but *here* it would crash
End Sub

How can you create subs and functions in VBA that will accept a null parameter?