Stop data entry if certain criteria exists

I have a time reporting database that has a form with a start and stop time.

When the employee selects their name from a combo box and if they haven't entered a stop time in their previous transaction I have a message box appearing.  But the problem is, the message box is appearing for any employee even if they don't have "un-ended" transactions.

Here is my code.  Can someone spot the problem for me?

    If IsNull(DMax("[StopTime]", "tblLaborTranx", "[EmployeeID] = " & Forms!frmEmployeeLaborTranx!txtEmployeeID)) Then
        MsgBox "You have not logged out of your last labor transaction. Please be sure to log out of it or see the front office for editing."
        DoCmd.Close acForm, "frmEmployeeLaborTranx", acSaveNo
        Exit Sub
    End If

I also have tried:

    If DMax("[StopTime]", "tblLaborTranx", "[EmployeeID] = " & Forms!frmEmployeeLaborTranx!txtEmployeeID) = "" Then
        MsgBox "You have not logged out of your last labor transaction. Please be sure to log out of it or see the front office for editing."
        DoCmd.Close acForm, "frmEmployeeLaborTranx", acSaveNo
        Exit Sub
    End If

But that doesn't work either.

--Steve