Six Operations: a natural transformation $i^! \to i^*$

Question

Let $i : Z \to X$ be the inclusion of a closed subset of sufficiently nice spaces to support the six operations. I'm reading a paper where the author says there is a natural transformation of functors $i^! \to i^*$. Here $i^!$ is the exceptional inverse image and $i^*$ the inverse image being the usual functors $D^b(X) \to D^b(Z)$ of derived categories of (constructible) sheaves. For the life of me I am not able to figure out where this natural transformation is coming from. I know that in the six functor formalism we always have a natural transformation $i_! \to i_*$ which is an isomorphism in this case as the map $i$ is proper. I've tried playing around with the various adjunction maps, but haven't gotten anywhere.