Can't Place Ladders on Observer Blocks

If you place down an observer block and try to place a ladder against it, nothing happens. This doesn't make sense, as observers act like normal blocks in every other way. They're not tile entities, they can be pushed by pistons, they conduct redstone, you can place signs on them, you can even place vines on them, etc....but you can't place ladders on them? Feels like a bug to me, no?