error: Uncaught (in promise) AssertionError

[goodmanworld]

I am a new Deno learner, I tried a tutorial code that uses MongoDB but I got the following error:

error: Uncaught (in promise) AssertionError
    throw new AssertionError(msg);
          ^
    at assert (https://deno.land/std@0.107.0/testing/asserts.ts:224:11)
    at MongoClient.database (https://deno.land/x/mongo@v0.27.0/src/client.ts:67:5)
    at file:///C:/Users/m/Desktop/Uproject/GuidApp/deno-survey/mongo.ts:6:19

As the project has many files, Let me know if is it needed to add some parts of project codes here?
But the whole project code can be find here: https://github.com/thecodeholic/deno-survey