How to prove that a kernel is not valid

Question

So basically I have two symmetric and positive semidefinite kernels k1 and k2. I'm trying to prove that a1(k1) - a2(k2) is not a valid kernel if a1 and a2 are real numbers > 0. Obviously subtraction does not work that same way that addition does with kernels. How can I give a concrete example to disprove this?