Why are edge fields in undolators necessary to comply with Maxwell?

In our lecture scripts there is a paragraph about the magnetic field of an undulator:

"Its [the undulators] magentic field can be described by
begin{equation}
vec{B}equiv-B_0 begin{pmatrix} 0 sin k_u z 0 end{pmatrix}
end{equation}
where $k_u$ is the undulator wave number
defined by $?_? = 2??_?$. Note that this definition injures Maxwell’s equation [$nablacdot vec B = 0$], but this does not
play a role here since „compensating“ edge fields exist that can be neglected for this definition."

How exactly does this B-field violate $nablacdot vec B = 0$?

begin{align}
nablacdot vec B &= partial_x cdot 0 + partial_y (-B_0 sin k_u z)+partial_z cdot 0
&= 0
end{align}
What am I missing? If the undulator was infinitely big, there would be no edge fields but the B-field would still be there. Then how can it violate Maxwell’s equations?