Comparing ObjectId's using redis and mongodb

I have run into some issues when comparing ObjectIds using mongoDB and redis.

I store MongoDB documents as JSON strings in redis and use JSON.parse when retrieving from redis, but this converts the ObjectIds to strings and I am having a hard time converting them back.

I am using NodeJS with mongoose, and have tried the following:

mongoose.Types.ObjectId(string_of_objectid)

but I get the error

Argument passed in must be a single String of 12 bytes or a string of 24 hex characters in hex format