Clearing cache on New Identity in Tor Browser 5.0a3 is throwing exceptions

Clearing the cache on New Identity I get

Torbutton: Unexpected error during offline cache clearing: [Exception... "Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsICacheService.evictEntries]"  nsresult: "0x80004001 (NS_ERROR_NOT_IMPLEMENTED)"  location: "JS frame :: chrome://torbutton/content/torbutton.js :: torbutton_do_new_identity :: line 1792"  data: no]