diff --git a/mmdet3d/models/dense_heads/point_rpn_head.py b/mmdet3d/models/dense_heads/point_rpn_head.py
index 71df2389e8..5561ce52db 100644
--- a/mmdet3d/models/dense_heads/point_rpn_head.py
+++ b/mmdet3d/models/dense_heads/point_rpn_head.py
@@ -343,6 +343,11 @@ def class_agnostic_nms(self, obj_scores, sem_scores, bbox, points,
             score_selected = obj_scores_nms[keep]
             cls_preds = sem_scores_nms[keep]
             labels = torch.argmax(cls_preds, -1)
+        else:
+            bbox_selected = bbox.tensor
+            score_selected = obj_scores.new_zeros([0])
+            labels = obj_scores.new_zeros([0])
+            cls_preds = obj_scores.new_zeros([0, sem_scores.shape[-1]])
 
         return bbox_selected, score_selected, labels, cls_preds