onBeforeUpdate - 'Not a single item' error

I have onBeforeAdd and onBeforeDelete methods on an ItemType to change a property depending on the status of that property on other instances of that ItemType. Those work fine. But when I tried to add an onBeforeUpdate method to detect when another property was changed, the 'Not a single item' error shows up. I'm also not able to use the apply() method on any item that is changed on the update method. I just need to know how to make a change to the item stick when using an Update server method.

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