findOne with fields causes insert on optional field

[BramVerb]

On the client side I do a Books.findOne({}, {fields: {anOptionalField:1}})
The anOptionalField in this my case is another object created using Class.create({...})

Then the I get the following error:

Error invoking Method '/Astronomy/insert' errorClass {
details: undefined,
error: 403,
errorType: "Meteor.Error",
isClientSafe: true,
message: "Inserting from the client is not allowed [403]",
reason: "Inserting from the client is not allowed"
}

And it does not throw the error when I use the following query:
Books.findOne({}, {fields: {anOptionalField:1}}, defaults: false)

To me it seems like a bug that a findOne query causes the client to try and insert something into the collection.

[lukejagodzinski]

You need to create reproduction repository for me to test it. I don't think it's Astronomy's problem. You probably have some error in your code.