Checkbox Value returns "undefined" in email body

Hello I'm trying to have a checkbox, when not checked display a message in the email body, but all I get is "undefined".

var WI="Inventory Control - This  includes Warehouse Inventory and Rental Inventory";

var cb1 =  this.getField("WI-U").value;

     if ( cb1 =="Off") WI.value;

var cToAddr ="Email@Company.com"

var cSubLine =""

var cBody = "Thank you for completing the file. \n\n"

+ cb1.value + "\n"

when the check box is off I need it to display plain text from var WI in the body of the email.

Any assistance is greatly appreciated.