I've just upgraded to version 3.3.0 from version 3.2.7 and receive the following exception:

TypeError: readPreference.equals is not a function
at executeWithServerSelection (<PATH_TO_SRC>/node_modules/mongodb/lib/operations/execute_operation.js:125:40)
at executeOperation (<PATH_TO_SRC>/node_modules/mongodb/lib/operations/execute_operation.js:92:16)
at AggregationCursor._initializeCursor (<PATH_TO_SRC>/node_modules/mongodb/lib/core/cursor.js:545:7)
at AggregationCursor._initializeCursor (<PATH_TO_SRC>/node_modules/mongodb/lib/cursor.js:191:11)
at nextFunction (<PATH_TO_SRC>/node_modules/mongodb/lib/core/cursor.js:748:10)
at AggregationCursor._next (<PATH_TO_SRC>/node_modules/mongodb/lib/core/cursor.js:202:5)
at fetchDocs (<PATH_TO_SRC>/node_modules/mongodb/lib/operations/to_array.js:27:14)
at ToArrayOperation.execute (<PATH_TO_SRC>/node_modules/mongodb/lib/operations/to_array.js:60:5)
at <PATH_TO_SRC>/node_modules/mongodb/lib/operations/execute_operation.js:109:26
at new Promise (<anonymous>)

The exception arises from using an aggregate function and calling .toArray() on the result:

unprocessedCompanyAggregate = await getCollection('company').aggregate([
   {
      '$match': {
         '_id' : createIDForDatabase(companyID)
      }
   },
   {
      '$project': {
         'financialPeriod': {
            '$arrayElemAt': ['$financialPeriods', financialPeriodsIndex]
         }
      }
   },
   {
      '$project': {
         'statementsName': '$financialPeriod.statementsType.name'
      }
   }
],{
   'session': session
}).toArray(); // toArray needed so converts the cursor for the aggregation to an array of documents.

The problem bit of mongodb code is in execute_operation.js:

const readPreference = operation.readPreference || ReadPreference.primary;
const inTransaction = operation.session && operation.session.inTransaction();
if (inTransaction && !readPreference.equals(ReadPreference.primary)) {
...
}

The only time this exception seems to occur is when operation.readPreference exists.

I'm using transactions, and the only time I'm setting readPreference is when starting a transaction:

session.startTransaction({
   'writeConcern': {
      'w': WRITE_CONCERN_W_MAJORITY,
      'j': WRITE_CONCERN_J_TRUE,
      'wtimeout': WRITE_CONCERN_WTIMEOUT_1000,
   },
   'readConcern': {
      'level': 'snapshot'
   },
   'readPreference': {
      'mode': 'primary'
   }
});

Where 

const WRITE_CONCERN_W_MAJORITY = 'majority';
const WRITE_CONCERN_J_TRUE = true;
const WRITE_CONCERN_WTIMEOUT_1000 = 1000;

 Any ideas what is going on?