Mongoose Schema default rand-tokens are the same

I have a UserSchema like this and I cannot seem to generate unique random activation_token's.

I am using rand-token for the generation. Found here.

 var UserSchema = new Schema({
        activation_token: {
            type: String,
            default: randToken.generate(64),
        },
        email: {
            type: String,
            unique: true,
            sparse: true
        },
        first_name: {
            type: String
        },
        last_name: {
            type: String
        }
});

Seemed to work fine, but when running unit tests with Mocha, all of the activation_token fields were the same. I initially thought this had to do with time, since that is probably what is used to generate the tokens. It was possible that for each new document, the timestamp was the same, so I ran some tests with a function that generated about 30 tokens one right after the other, and they were not similar.

Any ideas on what is going on here?

Here are some examples of the problem:

{
    "_id": {
        "$oid": "555dfd137c914edc1b41bbda"
    },
    "email": "oka@haek.io",
    "first_name": "Lenora",
    "last_name": "Aguilar",
    "date_added": {
        "$date": "2015-05-21T15:43:01.576Z"
    },
    "activation_token": "EyBNwu4vxOIXMzj7W5kVOeICfWwxfjXmHkz7ZPHLjkf0MU86QM2aIKNDyvI2YmTR",
    "__v": 0
},
{
    "_id": {
        "$oid": "555dfd107c914edc1b41bbd6"
    },
    "email": "ediuki@mu.edu",
    "first_name": "Eugene",
    "last_name": "Green",
    "date_added": {
        "$date": "2015-05-21T15:43:01.576Z"
    },
    "activation_token": "EyBNwu4vxOIXMzj7W5kVOeICfWwxfjXmHkz7ZPHLjkf0MU86QM2aIKNDyvI2YmTR",
    "__v": 0
}