Error, (in simplify/tools/_zn) too many levels of recursion

with the following input:

with(VectorCalculus);

X := r -> sqrt(2/Pi)*alpha*exp(-alpha^2*(r.r));

a_0:=a*<1/(2*sqrt(3) , 1/2>;

r:=<x,y>;

K:=<k_1, k_2>;

assume(alpha::real);

assume(alpha>0);
assume(k_1>0);

assume(k_2>0);

assume(k_1::real);

assume(k_2::real);

eq1:=int( X(r-a_0) , x=-infinity .. infinity);

Why is the ouput :

Error, (in simplify/tools/_zn) too many levels of recursion

I don't see what I'm doing wrong, help would be greatly appreciated;