Put a <NULL> in a SQL database

[BukHix]

I am building a front end for a shrink-wrapped accounting program. I have one feild in my app that can sometimes contain a value and sometimes not. The problem that I am having is that the shrink-wrapped program will not accept an empty string in the field of the database.

There has to be a value or it has to be a <NULL>. Empty strings such as "" cause it to error. Any ideas of how to accomplish this from my form?

In my AddRecord Function I have this but none of these worked.

Dim strCityTax As String
 
        If txtCityTax.Text = "" Then
            'strCityTax = vbNullString
            'strCityTax = Convert.DBNull
            'strCityTax = DBNull
            'strCityTax = vbNullString.ToString
        Else
            strCityTax = txtCityTax.Text
        End If