How can I upadate a decimal/numeric field with null?

When I edit a view, i want that a custom actions upadate a decimal field of the form.

I tried that but it doesn't work. I want to make this field "Result" empty.

void status.Rerun(int recID)
{
    thisRec = Sample_Receiving [ID == input.recID];

        thisRec.Result = null;

    info thisRec.Status;

Thank you