Datagridview Check Box Error Default

I have a Datagridview in which a user will input data for shifts they have worked. I have a Checkbox to tick whether they require an on call payment for the shift. It's unchecked Value is "N" and when checked the value is "Y". This works well.

However when the user leaves the check box at it's default state from load, when i come to send it to the database i get an exception. How can I get the default value to always be "N" even if the user hasn't altered the box?

Error Message: Object Reference Not set to an instance of an object.