I have a suite of tests for an API app that uses mongoDB, sometimes all the tests pass but sometimes I get this error.

test index/2 OK (WorkersWeb.Api.V1.CountyControllerTest)
     test/workers_web/controllers/api/v1/county_controller_test.exs:18
     ** (Protocol.UndefinedError) protocol Enumerable not implemented for nil. This protocol is implemented for: DBConnection.PrepareStream, DBConnection.Stream, Date.Range, File.Stream, Function, GenEvent.Stream, HashDict, HashSet, IO.Stream, List, Map, MapSet, Mongo.AggregationCursor, Mongo.Cursor, Mongo.SinglyCursor, Range, Stream
     code: assert Enum.count(json["counties"]) == 3
     stacktrace:
       (elixir) /home/boris/.kiex/builds/elixir-git/lib/elixir/lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir) /home/boris/.kiex/builds/elixir-git/lib/elixir/lib/enum.ex:153: Enumerable.count/1
       (elixir) lib/enum.ex:554: Enum.count/1
       test/workers_web/controllers/api/v1/county_controller_test.exs:27: (test)

The error doesn’t appear in the same place, is this some kind of race condition error where the tests is done before mongo creates the item to be tested?