In my opinion, clearing the target when using /assist on a PC/NPC with nothing targeted is the only logical choice. 
 
Leaving it at the last thing you had targeted on a failed /assist is only going to make you wonder whether your PC/NPC actually had that last thing targeted, too, similar to the problem of targeting oneself upon failure. 
 
The only consistent and unambiguous choice is for /assist to always set your target to that of your target's target, even when that's nothing at all. 
 
The only question I have after that is whether /assist has a maximum range. /target has a maximum range, but once you do have something targeted, you can keep that target as long as you're in the zone and you're both alive. 
 
Can you then /assist something that's across the zone but that you still have targeted? Or, does /assist have a range limit like /target, which is the way I would assume it to work? 
 
- Shendare 
		
	
		
		
		
		
		
		
		
		
		
	
		
		
	
	
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