diff --git a/egs/librispeech/ASR/conformer_ctc_bn_2d/conformer.py b/egs/librispeech/ASR/conformer_ctc_bn_2d/conformer.py
index 971fc578a..4fb8bef7f 100644
--- a/egs/librispeech/ASR/conformer_ctc_bn_2d/conformer.py
+++ b/egs/librispeech/ASR/conformer_ctc_bn_2d/conformer.py
@@ -668,8 +668,10 @@ class SimpleCausalEncoderLayer(nn.Module):
 
 
 class ReverseGrad(torch.autograd.Function):
+    @staticmethod
     def forward(ctx, x):
         return x
+    @staticmethod
     def backward(ctx, x_grad):
         return -x_grad
 
@@ -1770,6 +1772,7 @@ def _test_bidirectional_conformer():
     N = 10
     C = num_features
     feats = torch.randn(N, T, C)
+    feats.requires_grad = True
 
     tokens = _gen_rand_tokens(N)
     supervision = _gen_supervision(tokens)
@@ -1802,6 +1805,9 @@ def _test_bidirectional_conformer():
 
     print("self prediction logprob = ", self_prediction_logprob)
 
+    loss = -(decoder_logprob + reverse_decoder_logprob - self_prediction_logprob)
+    loss.backward()
+
 
 if __name__ == '__main__':
     _test_bidirectional_conformer()