ZeroDivisionError: division by zero

[JasonRichard]

I got ZeroDivisionError: division by zero from panopticapi/evaluation.py for empty prediction.
With tp + fp + fn == 0, self increament of n is skipped.
When all of categories.items() are skipped because empty prediction, n becomes zero and an error is reported.
Shouldn't n increase whenever category isthing (right after the first continue)?

def pq_average(self, categories, isthing):
    pq, sq, rq, n = 0, 0, 0, 0
    per_class_results = {}
    for label, label_info in categories.items():
        if isthing is not None:
            cat_isthing = label_info['isthing'] == 1
            if isthing != cat_isthing:
                continue
        iou = self.pq_per_cat[label].iou
        tp = self.pq_per_cat[label].tp
        fp = self.pq_per_cat[label].fp
        fn = self.pq_per_cat[label].fn
>>        if tp + fp + fn == 0:
>>            per_class_results[label] = {'pq': 0.0, 'sq': 0.0, 'rq': 0.0}
>>            continue
>>        n += 1
        pq_class = iou / (tp + 0.5 * fp + 0.5 * fn)
        sq_class = iou / tp if tp != 0 else 0
        rq_class = tp / (tp + 0.5 * fp + 0.5 * fn)
        per_class_results[label] = {'pq': pq_class, 'sq': sq_class, 'rq': rq_class}
        pq += pq_class
        sq += sq_class
        rq += rq_class

    return {'pq': pq / n, 'sq': sq / n, 'rq': rq / n, 'n': n}, per_class_results