I am using deep learning library keras and trying to stack multiple LSTM with no luck.
Below is my code

```
model = Sequential()
model.add(LSTM(100,input_shape =(time_steps,vector_size)))
model.add(LSTM(100))

```

The above code returns error in the third line `Exception: Input 0 is incompatible with layer lstm_28: expected ndim=3, found ndim=2`

The input X is a tensor of shape (100,250,50). I am running keras on tensorflow backend