error while merging summaries using tf.compat.v1.summary.merge_all()

For some reason, summary is None.

Error message

Code :

    def build_graph(self):        tf.logging.info('Building graph...')        t0 = time.time()        self._add_placeholders()        self._add_seq2seq()        self.global_step = tf.Variable(0, name='global_step', trainable=False)        if self._hps.mode == 'train'or self._hps.mode == 'pretrain':            self._add_train_op()            self._rollout()        self._summaries = tf.compat.v1.summary.merge_all()        tf.logging.info('Time to build graph: %i seconds', time.time() - t0)

I tried adding one tf.scalar_summary() before calling tf.compat.v1.summary.merge_all() but it didn't work.