Stargate DHD doesn't work after loading a save

I’m having a slight issue with the Stargate addon:
When I place a Stargate and DHD, it works fine. I can use the DHD to dial the gate, I can put an iris on the gate and open and close it.
However, if I save the game (singleplayer) and reload it later, I get the following error when I try to dial the gate, whether via DHD or at the gate: “entities\stargate_base\modules/events.lua:64: attempt to index field ‘Entity’ (a nil value)”. Also, when I try to use the iris or shield (whether by hotkey or button, both of which worked during the last session) I get “Wire error (Entity [103][stargate_iris]): entities/stargate_iris/init.lua:116: attempt to index field ‘Phys’ (a nil value)”
Has anyone else had this problem? What do I need to change to fix it?