Looking at what you have it appears that it should be working.
The only thing I can think of from the code you supply is the value of 'eref-' . $category[0]->slug . '' does not equal your field name, of the field for 'eref-' . $category[0]->slug . '' is not set.
Have you tried echoing the value of 'eref-' . $category[0]->slug . '' to see what it is?
