MongoDB Enterprise ruby-driver-rs:PRIMARY> db.runCommand({find:"foo",$readPreference:{}})
{
	"operationTime" : Timestamp(1594232502, 1),
	"ok" : 0,
	"errmsg" : "Missing expected field \"mode\"",
	"code" : 4,
	"codeName" : "NoSuchKey",
	"$clusterTime" : {
		"clusterTime" : Timestamp(1594232502, 1),
		"signature" : {
			"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
			"keyId" : NumberLong(0)
		}
	}
}

irb(main):002:0> c.database.command(find: "foo", '$readPreference': {})

localhost:14420 #1:1 | admin.find | STARTED | {"find"=>"foo", "$readPreference"=>{"mode"=>"primary"}, "$db"=>"admin", "lsid"=>{"id"=><BSON::Binary:0x1020 type=uuid data=0x6e42f7d4fd24479a...>}}
D, [2020-07-08T14:22:45.203680 #7382] DEBUG -- : MONGODB | [10] localhost:14420 | admin.find | FAILED | BSON field 

'OperationSessionInfo.$readPreference' is a duplicate field (40413) | 0.000546408s
Traceback (most recent call last):
       16: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/op_msg_or_command.rb:26:in `execute'
       15: from /home/w/apps/ruby-driver/lib/mongo/server.rb:425:in `with_connection'
       14: from /home/w/apps/ruby-driver/lib/mongo/server/connection_pool.rb:590:in `with_connection'
       13: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/op_msg_or_command.rb:28:in `block in execute'
       12: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/executable.rb:44:in `execute'
       11: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/executable.rb:44:in `tap'
       10: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/executable.rb:45:in `block in execute'
        9: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:26:in `validate_result'
        8: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:82:in `unpin_maybe'
        7: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:27:in `block in validate_result'
        6: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:43:in `add_error_labels'
        5: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:28:in `block (2 levels) in validate_result'
        4: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:96:in `add_server_diagnostics'
        3: from /home/w/apps/ruby-driver/lib/mongo/operation/shared/response_handling.rb:29:in `block (3 levels) in validate_result'
        2: from /home/w/apps/ruby-driver/lib/mongo/operation/result.rb:289:in `validate!'
        1: from /home/w/apps/ruby-driver/lib/mongo/operation/result.rb:321:in `raise_operation_failure'
Mongo::Error::OperationFailure (BSON field 'OperationSessionInfo.$readPreference' is a duplicate field (40413) (on localhost:14420))