keras: model.fit ValueError: I/O operation on closed file

Any idea what could be causing this error? I’ve been trying to solve this for a week. Thanks in advance.

Train on 60816 samples, validate on 15204 samples
Epoch 1/20
60816/60816 [==============================] - 19s - loss: 0.1665 - acc: 0.9597 - val_loss: 0.1509 - val_acc: 0.9605
Epoch 2/20
31200/60816 [==============>...............] - ETA: 8s - loss: 0.1583 - acc: 0.9600
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-e3590f9fe5e6> in <module>()
     16 
     17 my_model = model.fit(train_x, train_y, batch_size=100, nb_epoch=20,                       
---> 18                      show_accuracy=True, verbose=1, validation_data=(test_x, test_y))
     19 score = model.evaluate(test_x, test_y, show_accuracy=True, verbose=0)
     20 print('Test loss:', score[0])

/usr/local/lib/python2.7/dist-packages/keras/models.pyc in fit(self, X, y, batch_size, nb_epoch, verbose, callbacks, validation_split, validation_data, shuffle, show_accuracy, class_weight, sample_weight)
    644                          verbose=verbose, callbacks=callbacks,
    645                          val_f=val_f, val_ins=val_ins,
--> 646                          shuffle=shuffle, metrics=metrics)
    647 
    648     def predict(self, X, batch_size=128, verbose=0):

/usr/local/lib/python2.7/dist-packages/keras/models.pyc in _fit(self, f, ins, out_labels, batch_size, nb_epoch, verbose, callbacks, val_f, val_ins, shuffle, metrics)
    284                     batch_logs[l] = o
    285 
--> 286                 callbacks.on_batch_end(batch_index, batch_logs)
    287 
    288                 epoch_logs = {}

/usr/local/lib/python2.7/dist-packages/keras/callbacks.pyc in on_batch_end(self, batch, logs)
     58         t_before_callbacks = time.time()
     59         for callback in self.callbacks:
---> 60             callback.on_batch_end(batch, logs)
     61         self._delta_ts_batch_end.append(time.time() - t_before_callbacks)
     62         delta_t_median = np.median(self._delta_ts_batch_end)

/usr/local/lib/python2.7/dist-packages/keras/callbacks.pyc in on_batch_end(self, batch, logs)
    168         # will be handled by on_epoch_end
    169         if self.verbose and self.seen < self.params['nb_sample']:
--> 170             self.progbar.update(self.seen, self.log_values)
    171 
    172     def on_epoch_end(self, epoch, logs={}):

/usr/local/lib/python2.7/dist-packages/keras/utils/generic_utils.pyc in update(self, current, values)
     59             prev_total_width = self.total_width
     60             sys.stdout.write("\b" * prev_total_width)
---> 61             sys.stdout.write("\r")
     62 
     63             numdigits = int(np.floor(np.log10(self.target))) + 1

/usr/local/lib/python2.7/dist-packages/ipykernel/iostream.pyc in write(self, string)
    315 
    316             is_child = (not self._is_master_process())
--> 317             self._buffer.write(string)
    318             if is_child:
    319                 # newlines imply flush in subprocesses

ValueError: I/O operation on closed file