Aux channels

Solo

?, I am mixing a song with a track’s (#5) output to Aux1 then to the master channel. I can not “solo” another track unless I mute Aux1 first. Could someone please explain? I could just send it to a group and complish what I am trying to do but I am just curious. I can not solo the Aux channel either. I am running the latest (as of 7_10_12) v7 32bit on win.