unsupported type for timedelta minutes component: NoneType

Initially, it ran well but when I retry to run it, it gives an error in [gsuite-exporter](https://github.com/GoogleCloudPlatform/professional-services/tree/master/tools/gsuite-exporter#gsuite-exporter).

> sync_all(
credentials_path="cerd.json",
admin_user='admin@domain.com',
api='reports_v1',
applications=['login', 'drive', 'token'],
project_id='',
exporter_cls='stackdriver_exporter.StackdriverExporter'
)

below error:

> /gsuite_exporter/cli.py in sync_all(admin_user, api, applications, project_id, exporter_cls, credentials_path, offset)
70 start_time = None
71 else:
---> 72 start_time = (parser.parse(last_ts) - timedelta(minutes=offset)).isoformat()
73 exporter_dest = exporter.get_destination(app)
74 logger.info(
TypeError: unsupported type for timedelta minutes component: NoneType