Cannot change the value of a field to zero lenght using Visual Basic 6.0 and a Microsoft Access 2000 Database

I am using Visual Basic 6.0 and a Microsoft Access 2000 Database.  I am using a ADO data control.  Once I write to a field I cannot delete the value in the field. For instance if I enter a value in the field  phone number and which to delete it because the patient has no phone number or has had it disconnected, I get the following error message: [Microsoft][ODBC Microsoft Access Driver] Field'demograpTbl.PhoneNumber' cannot be zero-length string.  The field property is set so that PhoneNumber is not a require field.