AssertionError: output is not in graph

python -m tf2onnx.convert \
--checkpoint ./ckpt/ner.ckpt-99.meta \
--output  model.onnx \
--inputs  inputs_charID:[1,70],inputs_langID:[1,70],inputs_textID:[1,70],inputs_posID:[1,70],inputs_wlID:[1,70],inputs_wpID:[1,70],inputs_cwsID:[1,70] \
--outputs output:[1,-1] \
--opset 11
raise an error :
assert d in name_to_node, "%s is not in graph" % d
AssertionError: output is not in graph