Inverter and reverse polarity

I'm trying to get my inverter to power up my shore power system. (just temporary). I'm using a short cord from inverter to shore power receptacle and the reverse polarity lights up on the panel. I have reversed the wires on the extension cord but it makes no difference. I then tried running a ground wire from the inverter chassis to the ground wire on the shore power receptacle and then I tried it on the common still the light comes on. I'm sure its something very simple but I can't figure it out.