DP-coloring was introduced by Dvořák and Postle and is a generalization of proper coloring. For any graph $G$, let $χ(G)$ and $χ_{DP}(G)$ denote the chromatic number and the DP-chromatic number of $G$ respectively. In this article, we show that $χ_{DP}(G \vee K_s)=χ(G \vee K_s)$ holds for $s=\left \lceil \frac{4(k+1)m}{2k+1} \right \rceil \le \lceil 2.4m\rceil$, where $k=χ(G)$, $m=|E(G)|$ and $G \vee K_s$ is the join of $G$ and the complete graph $K_s$. Hence $Z_{DP}(n)\le n^2-(n+3)/2$ holds for every integer $n \ge 2$, where $Z_{DP}(n)$ is the minimum natural number $s$ such that $χ_{DP}(G \vee K_s)=χ(G \vee K_s)$ holds for every graph $G$ of order $n$. Our result improves the best current upper bound $Z_{DP}(n)\le 1.5n^2$ due to Bernshteyn, Kostochka and Zhu.