Rogue update query being run without supporting indexes

I haven't identified yet where this query is coming from but for the reference:

gitlabhq_production=# explain UPDATE "issues" SET "last_edited_by_id" = 0 WHERE "issues"."deleted_at" IS NULL AND "issues"."last_edited_by_id" = 0;
                             QUERY PLAN                              
---------------------------------------------------------------------
 Update on issues  (cost=0.00..1080953.14 rows=46 width=776)
   ->  Seq Scan on issues  (cost=0.00..1080953.14 rows=46 width=776)
         Filter: ((deleted_at IS NULL) AND (last_edited_by_id = 0))
(3 rows)