pt_prepare_proxy_list_for_config_read: Assertion unconfigured_proxies_n == 0 failed

Tor dies with

Feb 11 08:34:46.668 [Error] pt_prepare_proxy_list_for_config_read(): Bug: transports.c:1221: pt_prepare_proxy_list_for_config_read: Assertion unconfigured_proxies_n == 0 failed; aborting.

Steps to reproduce:

1. configure your tor to use a managed ClientTransportPlugin, e.g.

ClientTransportPlugin obfs2 exec /path/to/obfsproxy --managed

2. hup your Tor twice in quick succession