view_merger error during rebalance_out query tests

Have a view test that performs a rebalance out of all servers while running queries, eventually giving this response:

{"error":"{nocatch,{error,{case_clause,{216,\n [<<\"employee-44-a7d631e\">>,\n <<\"44@couchbase.com\">>]}}}}","reason":"[

{couch_set_view,get_group,3}

,\n

{couch_set_view,get_map_view,5}

,\n

{couch_view_merger,prepare_set_view,4}

,\n

{couch_view_merger,map_set_view_folder,5}

]"}

query:
http://10.1.2.130:8091/couchBase/default/_design/dev_test_view-08ddf55/_view/dev_test_view-08ddf55?debug=true&connection_timeout=60000&full_set=true&stale=false

reproducible in ui (see screen)