Shell incorrectly issues first query in legacy read mode

MongoDB shell version: 3.2.1

connecting to: test

> db.version()

3.2.1

> db.foo.find().sort([{a: 1}, {b: 1}])  // Executes with OP_QUERY legacy reads.

{ "_id" : ObjectId("56b0fbb44739a301b71f0e73") }

{ "_id" : ObjectId("56b0fd134739a301b71f0e78"), "a" : -1, "b" : 3 }

{ "_id" : ObjectId("56b0fd164739a301b71f0e79"), "a" : -1, "b" : 7 }

{ "_id" : ObjectId("56b0fd0c4739a301b71f0e74"), "a" : 1, "b" : 1 }

{ "_id" : ObjectId("56b0fd0f4739a301b71f0e76"), "a" : 1, "b" : 3 }

{ "_id" : ObjectId("56b0fd0e4739a301b71f0e75"), "a" : 1, "b" : 5 }

{ "_id" : ObjectId("56b0fd114739a301b71f0e77"), "a" : 2, "b" : 3 }

> db.foo.find().sort([{a: 1}, {b: 1}])  // Executes with read commands.

Error: error: {

	"waitedMS" : NumberLong(0),

	"ok" : 0,

	"errmsg" : "Failed to parse: sort: [ { a: 1.0 }, { b: 1.0 } ]. 'sort' field must be of BSON type Object.",

	"code" : 9

}