BibTeX does not have the right entry for preprints. This is a hack for producing the correct reference:
@Booklet{EasyChair:172,
author = {Chiaki Sakama and Hien Nguyen and Taisuke Sato and Katsumi Inoue},
title = {Partial Evaluation of Logic Programs in Vector Spaces},
howpublished = {EasyChair Preprint no. 172},
doi = {10.29007/9d61},
year = {EasyChair, 2018}}