Cannot filter Members/PublicData on slug

I am using the code and information documented here to query the “Members/PublicData” collection

The documentation says that you can filter using “.eq” on the slug, but if I do that I get an error (the error code is not available).

		wixData.query("Members/PublicData") 
		.eq('slug', 'username123' ) 
		.find() 

If I filter on another field, example “nickname” using “.eq” then it works.

Why does the filter not work with the slug field as documented ?