Command Button disabled until checkbox selected

Good Morning Folk

I have a userform that will open when the workbook opens. It has two checkboxes and a command button. I am looking for the command button to be disabled until one of the checkboxes is selected.

The checkboxes are working ok however the command button is enabled at all times and selecting it without selecting a checkbox throws up an error in the 'cmdenter.Enabled = False' line

This is the code I have so far, can someone point out where I have went wrong,, many thanks

Sub Workbook_Open()

    frmopen.Show

    If cboxincident Then
        cmdenter.Enabled = True
    End If
    If cboxcapa Then
        cmdenter.Enabled = True
    End If
        If Not cboxincident And Not cboxcapa Then
            cmdenter.Enabled = False
    End If

End Sub

Many thanks for your help