diff --git a/ppdet/modeling/post_process.py b/ppdet/modeling/post_process.py
index 78aeacf1c74..a66ba76a8e7 100644
--- a/ppdet/modeling/post_process.py
+++ b/ppdet/modeling/post_process.py
@@ -160,6 +160,9 @@ def get_pred(self, bboxes, bbox_num, im_shape, scale_factor):
         else:
             # simplify the computation for bs=1 when exporting onnx
             scale_y, scale_x = scale_factor[0][0], scale_factor[0][1]
+            # TODO(PIR): indexing yields 0-d tensors which concat rejects.
+            scale_y = paddle.unsqueeze(scale_y, 0)
+            scale_x = paddle.unsqueeze(scale_x, 0)
             scale = paddle.concat(
                 [scale_x, scale_y, scale_x, scale_y]).unsqueeze(0)
             self.origin_shape_list = paddle.expand(origin_shape,
diff --git a/ppdet/modeling/proposal_generator/rpn_head.py b/ppdet/modeling/proposal_generator/rpn_head.py
index 456a9322fcd..b334437f046 100644
--- a/ppdet/modeling/proposal_generator/rpn_head.py
+++ b/ppdet/modeling/proposal_generator/rpn_head.py
@@ -242,7 +242,7 @@ def _gen_proposal(self, scores, bbox_deltas, anchors, inputs):
 
         if self.export_onnx:
             output_rois = [onnx_topk_rois]
-            output_rois_num = paddle.shape(onnx_topk_rois)[0]
+            output_rois_num = paddle.shape(onnx_topk_rois)[0:1]
         else:
             output_rois = bs_rois_collect
             output_rois_num = bs_rois_num_collect