In blockchain pool mining, Pay-Per-Last-N-Shares (PPLNS) is one of the most commonly used reward mechanisms in practice, in which the mining pools will distribute the reward among the miners whose reported shares fall in the last N shares, according to the proportion of the number of shares of the miner in the last N shares. In the PPLNS mechanism, miners' reporting strategies may impose significant impacts to their rewards via determining the number of shares appeared in the last N shares. As such, how to strategically report their shares to the pool has become an important decision faced by miners. In this paper, we study the share reporting problem in PPLNS pools, and establish a share reporting model for a miner to optimize his/her rewards. We also propose a novel hybrid share reporting strategy for the miner based on our model, and design computational experiments to evaluate our hybrid share reporting strategy. The experimental results show that the hybrid strategy outperforms two baseline share reporting strategies commonly used in practice. This work is the first attempt to study the share reporting issue faced by miners in PPLNS pools, and it can provide important managerial insights for blockchain pool miners when making their share reporting decisions.