Incorrect simplification

[Alexmaster350]

If I define f(x):=x*(a/pi)^(1/4)*e^(-a*x^2),

then execute f(x)*f''(x)

and then simplify(ans),

the result given is incorrect (plus sign where there should be a minus sign - this is easily seen by substituting e.g. x=1,a=1). No variables have been defined - this occurs even after a fresh restart. What could be causing this?

Thanks.