ruby-driver-rs:PRIMARY> db.test.explain().aggregate([])
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {
					
				},
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "ruby-driver.test",
					"indexFilterSet" : false,
					"parsedQuery" : {
						
					},
					"winningPlan" : {
						"stage" : "COLLSCAN",
						"direction" : "forward"
					},
					"rejectedPlans" : [ ]
				}
			}
		}
	],
	"ok" : 1,
	"operationTime" : Timestamp(1554485036, 1),
	"$clusterTime" : {
		"clusterTime" : Timestamp(1554485036, 1),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	}
}

ruby-driver-rs:PRIMARY> db.test.explain().aggregate([])
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "ruby-driver.test",
		"indexFilterSet" : false,
		"parsedQuery" : {
			
		},
		"queryHash" : "8B3D4AB8",
		"planCacheKey" : "8B3D4AB8",
		"optimizedPipeline" : true,
		"winningPlan" : {
			"stage" : "COLLSCAN",
			"direction" : "forward"
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "speed",
		"port" : 27741,
		"version" : "4.1.9-283-g1b75c7b9b1",
		"gitVersion" : "1b75c7b9b1295fa8a94552da7f2c63cbc2cc2105"
	},
	"ok" : 1,
	"$clusterTime" : {
		"clusterTime" : Timestamp(1554484992, 1),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	},
	"operationTime" : Timestamp(1554484992, 1)
}

 1) ./spec/mongo/collection/view/aggregation_spec.rb[1:4:1]

Mongo::Collection::View::Aggregation#explain executes an explain

NoMethodError: undefined method `[]' for nil:NilClass

/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/operation/aggregate/result.rb:68:in `documents'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/cursor.rb:298:in `process'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/cursor.rb:123:in `each'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/collection/view/iterable.rb:45:in `each'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/collection/view/aggregation.rb:88:in `first'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/lib/mongo/collection/view/aggregation.rb:88:in `explain'
/data/mci/a01d1b690953c11a290f354659ef7fdc/src/spec/mongo/collection/view/aggregation_spec.rb:224:in `block (3 levels) in <top (required)>'