CameraPick is fine if there is no entity picked. You'll get a MAV if you attempt to do anything to the picked entity when no entity was picked, so code like Otacon suggests. Check if anything was picked before running your code. Depending on what you're doing of course this would probably be more useful (but amounts to the exact same thing:)
hit=CameraPick(foo)
If hit<>0
;do stuff
EndIf
hit=CameraPick(foo)
If hit<>0
;do stuff
EndIf