Sequelize destroy method is returning NaN instead of the number of rows deleted

I am trying to use the destroy instance method as below, however I keep receiving NaN instead of the number of rows deleted.

const Sequelize = require('sequelize');
const sequelize = new Sequelize(options);

// insert some records, etc

sequelize.destroy({ truncate: true }).asCallback((err, deleted) => {

    if (err) {
        throw err;
    }

    console.log(deleted);
    // Always prints NaN
});

Any help would be appreciated!

Thanks

via simon-p-r