[eluser]naren_nag[/eluser]
SOLVED: I has called my controller Article as well. That was the problem. Basically changing the controller's name was all it took.
// Create the Category object before the save
// THIS is what is giving the error
// If I comment out the two lines below, I can't save, but I can var_dump
// and get no errors.
$categoryObject = new Category();
$categoryObject->get_by_id($category_id);