MXNET Gluon Error when calling two consecutive backward - Check failed: type_ != nullptr

Question

I need to call backward function for two different models, separately.

# Loss_1
#----------------------



Losses_1 = []
with autograd.record():
    
    
   for batch_per_gpu in batch
es:        
       pred = model_1(batch_per_gpu)
        
       loss = loss_func(pred, truth)
        
       losses_1.append(loss)
   mx.autograd.backward(losses_1)


model_1_trainer.allreduce_grads()
model_1_trainer.update()



#Loss_2
#----------------------

Losses_2 = []
with autograd.record():
    
    
   for batch_per_gpu in batches:
        
       pred = model_2(batch_per_gpu)
        
       loss = loss_func(pred, truth)

       losses_2.append(loss)
   mx.autograd.backward(losses_2)

model_2_trainer.allreduce_grads()

model_2_trainer.update()

I get this error on the second call of the backward function:

Check failed: type_ != nullptr: The any container is empty requested=N5mxnet10Imperative6AGInfoE

Does anyone know how to fix this?

My MXNET version is 1.6.0