collection.find(...).toArray is not a function?

I’ve implemented the example code, however when the resolver hits the data source, this error pops up

TypeError: collection.find(...).toArray is not a function
      at _dataloader.default.ids (E:\test-graphql\node_modules\apollo-datasource-mongodb\dist\cache.js:31:6)    
      at dispatchQueueBatch (E:\test-graphql\node_modules\apollo-datasource-mongodb\node_modules\dataloader\index.js:246:22)
      at dispatchQueue (E:\test-graphql\node_modules\apollo-datasource-mongodb\node_modules\dataloader\index.js:233:5)
      at E:\test-graphql\node_modules\apollo-datasource-mongodb\node_modules\dataloader\index.js:70:20
      at process._tickCallback (internal/process/next_tick.js:61:11)

This is how I create the data source in it’s own file

const { MongoDataSource } = require("apollo-datasource-mongodb");

class ProductPages extends MongoDataSource {
  getProductPage(id) {
    return this.findOneById(id);
  }
}

exports.ProductPagesDataSource = new ProductPages(ProductPageModel);

This is how I add it to the ApolloServer

const { ProductPagesDataSource } = require("../graphql/schema/schema");
const app = new ApolloServer({
  ...
  dataSources: () => ({
    productPages: ProductPagesDataSource
  })
});

And in the resolver I call it like this (I use graphql-compose with graphql-compose-mongoose for my schema, but that shouldn’t be an issue since I can make it work using the DataLoader package)

schemaComposer.Query.addFields({
  page: ProductPageSchema.get("$findById").wrapResolve(next => rp => {
    return rp.context.dataSources.productPages.getProductPage(rp.args._id);
  })
});

What am I doing wrong?