Why is the zero vector not in the set of polynomials of the form p(t) = a + t^2 , where a is in R?

Why is the zero vector not in the set of polynomials of the form p(t) = a + t^2 , where a is in R?

I'm trying to prove that it's not a subspace and I know the answer lies in the fact that it doesn't contain the zero vector. I just don't understand how it doesn't.