[SalesForce] EmailMessage query returns ‘No such column’

I am trying to query emailMessage objects related to a custom object. When I try the query in the Developer console query editor it returns results, also works in workbench. However when I use the same query in an Apex class it gives an error.
The query is:

SELECT Id FROM EmailMessage WHERE RelatedToId = :customObject.Id 

And the error is:

No such column 'RelatedToId' on entity 'EmailMessage'.

The same happens with:

SELECT Id, RelatedToId FROM emailMessage

Also, probably related as is referencing the same relationship field, the following query isn't working:

SELECT (SELECT Id FROM Emails) FROM Custom_Object__c 

The background to this is trying to attach a file to the emailMessage, I attach in the apex class and it is sent with email however the emailMessage record doesn't have the attachment, is there an option to attach it, I can't find any, and this would solve my problem although I still can't understand why the above isn't working.