[SalesForce] Compile Error: sObject type ‘Custom__c’ is not supported in Managed Package

I'm creating an apex class that implements Schedulable and in the execute it queries a Custom Object but I can't understand why I'm getting this error when trying to query. Some other related questions (1, 2, 3)

• The object is created.
• User profile has access to the object and fields.
• There are no classes called from the new apex class I'm creating.

Code:

global class PopulatePreferences Field implements Schedulable {
    public static String CRON_EXP = '0 30 10 * * ?';

    global static String scheduleIt() {
        PopulatePreferences sm = new PopulatePreferences();
        return System.Schedule('Populate Preferred Artists Field', CRON_EXP, sm);
    }

    global void execute(SchedulableContext sc) {
        List<Custom__c> preferences = [SELECT Prefered__r.Name, Client__c FROM Custom__c limit 10000];

        List<Contact> contactsToUpdate = new List<Contact>();

        for (Contact c : [SELECT preferences__c FROM Contact limit 10000]) {
            for (Custom__c f : preferences) {
                if (f.Client__c == c.Id) {
                    if (c.preferences__c != null)
                        c.preferences__c += '; ' + f.Prefered__r.Name;
                    else    
                        c.preferences__c = f.Prefered__r.Name;
                }
            }
            contactsToUpdate.add(c);
        }
        update contactsToUpdate;
    }
}

When I run this code in our developer org it runs as expected, the problem is in the clients org where the Managed Package is installed.

Why could I be getting this error?