TypeError: Expected Operation, Variable, or Tensor, got 0

[VinayNitmz]

while running this code model.fit i am getting error :
TypeError: Expected Operation, Variable, or Tensor, got 0

" history=model.fit([x_tr,y_tr[:,:-1]], y_tr.reshape(y_tr.shape[0],y_tr.shape[1], 1)[:,1:] ,epochs=50,callbacks=[es],batch_size=128, validation_data=([x_val,y_val[:,:-1]], y_val.reshape(y_val.shape[0],y_val.shape[1], 1)[:,1:])) "

TypeError: Expected Operation, Variable, or Tensor, got 0