diff --git a/ppdet/utils/voc_eval.py b/ppdet/utils/voc_eval.py
index 496f7b42394ce18a8e1c8ab612fa4620aca7b722..67e66e4c683aaf7d5af2fd352817daf1ad3576d4 100644
--- a/ppdet/utils/voc_eval.py
+++ b/ppdet/utils/voc_eval.py
@@ -81,7 +81,8 @@ def bbox_eval(results,
             for i in range(len(gt_boxes)):
                 gt_box = gt_boxes[i]
                 gt_label = gt_labels[i]
-                difficult = difficults[i]
+                difficult = None if difficults is None \
+                                else difficults[i]
                 bbox_num = bbox_lengths[i]
                 bbox = bboxes[bbox_idx: bbox_idx + bbox_num]
                 gt_box, gt_label, difficult = prune_zero_padding(