Mongodb Javascript: BSON element “cursor” is missing on collection.findOne()

I have a function in Typescript that checks if the user’s email already exist, and returns it as a boolean value. The catch is that whenever the request gets sent via a REST Client ( Postman/ Thunder Client) Or it is executed via unit test, the function runs perfectly. However, as soon as the client sends the exact same request, I get the following error:

MongoUnexpectedServerResponseError: BSON element "cursor" is missing
    at CursorResponse.get (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:88:13)
    at CursorResponse.get cursor [as cursor] (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:216:17)
    ... 6 lines matching cause stack trace ...
    at async FindCursor.next (C:RepositoriesUAI-expressnode_modulesmongodbsrccursorabstract_cursor.ts:398:7)
    at async Collection.findOne (C:RepositoriesUAI-expressnode_modulesmongodbsrccollection.ts:506:17) {
  [Symbol(errorLabels)]: Set(0) {},
  [cause]: BSONError: BSON element "cursor" is missing
      at CursorResponse.get (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolon_demanddocument.ts:275:15)
      at CursorResponse.get (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:86:20)
      at CursorResponse.get cursor [as cursor] (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:216:17)
      at CursorResponse.get batch [as batch] (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:254:25)
      at CursorResponse.get batchSize [as batchSize] (C:RepositoriesUAI-expressnode_modulesmongodbsrccmapwire_protocolresponses.ts:262:17)
      at FindCursor._initialize (C:RepositoriesUAI-expressnode_modulesmongodbsrccursorfind_cursor.ts:75:33)
      at processTicksAndRejections (node:internal/process/task_queues:95:5)
      at async FindCursor.cursorInit (C:RepositoriesUAI-expressnode_modulesmongodbsrccursorabstract_cursor.ts:684:21)
      at async FindCursor.fetchBatch (C:RepositoriesUAI-expressnode_modulesmongodbsrccursorabstract_cursor.ts:720:7)
      at async FindCursor.next (C:RepositoriesUAI-expressnode_modulesmongodbsrccursorabstract_cursor.ts:398:7)

Here’s the function that causes the error:

function createUser(req: Request, res: Response) {
  const body = req.body;
const db = await connectToDb();
    const userProvider = new UserProvider(db);
    const userExists = await userProvider.userExists(body.email);
    if (userExists) {
      return res.status(400).send("User already exists");
    }
    const result = await userProvider.createUser(body);
    return res.json(result);
}

Here’s the code that checks if users exists:

class UserProvider {

    constructor(db: Db) {
        super(db.collection('dashboard_users'));

    }

    async userExists(email: string): Promise<boolean> {
        const user = await this.collection.findOne({ email }, {});
        return !!user;
    }

}

Here’s the client code, using Tanstack React Mutation and axios

type UserMutationFn = (data: any) => Promise<any>;

export const createAdminUser: UserMutationFn = async (data: any) => secureAPIPOST(`/users/admin`, data);
export const createUser: UserMutationFn = async (data: any) => secureAPIPOST(`/users`, data);
export const deleteUser: UserMutationFn = async (data: any) => secureAPIDELETE(`/users`, null, data);
export const updateUser: UserMutationFn = async (data: any) => secureAPIPOST(`/users`, data);


export default function useUserMutation(mutationFn: UserMutationFn) {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn,
    onError: (error) => {
      console.error(error);
    },
    onSuccess: () => {
      // invalidate dashboard users query
      queryClient.invalidateQueries({
        queryKey: ['dashboard-users']
      });
    }
  });
}

And lastly, here’s the abstract implemetnation of PaginatedProvider

import { Collection } from "mongodb";

export abstract class PaginatedProvider<T> {

    limit = 10;
    collection: Collection;


    constructor(collection: Collection) {
        this.collection = collection;
    }
}

A few notes to keep in mind:

• This is not the only provider, every other provider that I have created works perfeclty
• This function also runs with no issues when called from a client
• No CORS-related issues have been spotted
• I could not find any documentation for this specific error

Any help is greatly appreciated