I think the entity report says something like 'the brush entity (e.g.) func_door is empty' and then if you check the details it says 'entity has no solids' or something like that.
It happened to me in my map. It occurs when you tie a brush to an entity and then delete the brush but the entity will sometimes remain there without any brush for it to be tied to.
...and that's the bottom line because Mate de Vita said so.