diff --git a/mmdet3d/structures/ops/transforms.py b/mmdet3d/structures/ops/transforms.py
index 8e9f7006ac..1a698d9f4c 100644
--- a/mmdet3d/structures/ops/transforms.py
+++ b/mmdet3d/structures/ops/transforms.py
@@ -37,11 +37,12 @@ def bbox3d2roi(bbox_list):
     """
     rois_list = []
     for img_id, bboxes in enumerate(bbox_list):
+        img_inds = bboxes.new_full((bboxes.size(0), 1), img_id)
         if bboxes.size(0) > 0:
-            img_inds = bboxes.new_full((bboxes.size(0), 1), img_id)
             rois = torch.cat([img_inds, bboxes], dim=-1)
         else:
             rois = torch.zeros_like(bboxes)
+            rois = torch.cat([img_inds, rois], dim=-1)
         rois_list.append(rois)
     rois = torch.cat(rois_list, 0)
     return rois