diff --git a/panopticapi/evaluation.py b/panopticapi/evaluation.py
index 26b4970..7857162 100755
--- a/panopticapi/evaluation.py
+++ b/panopticapi/evaluation.py
@@ -178,6 +178,7 @@ def pq_compute_multi_core(matched_annotations_list, gt_folder, pred_folder, cate
     pq_stat = PQStat()
     for p in processes:
         pq_stat += p.get()
+    workers.close()
     return pq_stat