Excel userform checkboxes null

I have created a userform and added a chekcbox called 'DSSSContact'. I have written some code to check the contents of the userform before saving in the following sub:

Private Sub SaveandClose_Click()

When the form is initialized the checkbox is set greyed out but ticked - which I think means the value is null. The SavandClose sub code includes the following which I thought would make sure that checkbox is either set to true or false:

If DSSSContact = Null Then
MsgBox "DSSS Contact Test"
Exit Sub
End If

However when I save and close the form using the save and close command button without having made a selection on the checkbox the form saves the contents without triggering the message box. All the other checks that I have created to verify that textboxes and combo boxes are populated are working fine. I have also added the following code within the same sub:

Debug.Print Me.DSSSContact.Value

This prints null in the Immediate window

Any help much appreciated. Thanks