WinForms event handler possible NullReferenceException

I have an event handler that is used by multiple controls. within the handler is a series of ifs checking which control is the "sender" (as shown below).

else if (sender == socialSecurityNo)
	CremationSSN.Text = socialSecurityNo.Text;
else if (sender == CremationSSN)
	socialSecurityNo.Text = CremationSSN.Text;

Why is ReSharper telling in the first if that "socialSecurityNo" is possibly null (it's obviously not since it's the sender). Same thing with the second if - it's highlight with a blue squiggle and listed as possibly null.