Invariant yield of $\mathrm{Z} \rightarrow \mu^+\mu^-$ divided by $\left< T_{\rm AA} \right>$ in the rapidity range $2.5 <\ycms^{\mu\mu} <4.0$ for three centrality classes in Pb--Pb collisions at \fivenn. The uncorrelated systematic uncertainties are reported as filled boxes. The open boxes indicate the quadratic sum of correlated and uncorrelated systematics. The results are compared with the free-nucleon PDF prediction (CT14~) and with calculations with the centrality-dependent EPS09s nPDFs ~.