Negative value of word-level bounding-boxes in gt.mat

I downloaded the Pre-generated dataset (about 800K images), find that there some Negative Values in gt.mat For example, in the annotation of ['105/lobby+hotel_18_39.jpg'], the bounding-box of word "you" is annotated as "302, 47, 377, -1, 387, 14, 312, 63", you can find the full annotation in gt_mat["wordBB"][0][791867] to check it. I wanna know what the negative value in bbox annotation mean?