SharedObject data and..............Arrays! What the

The player buys engines these are added to an array called _model.purchasedEngines. During save i’m trying:

sharedObject.data.purchasedEngines = _model.purchasedEngines;

and during load I’m trying:

_model.purchasedEngines = sharedObject.data.purchasedEngines;

but when the magic happens I’m getting:

                   Error #1069: Property undefined not found on BaseScreen and there is no default value.

directed at this line:

var peMc:MovieClip =              asBaseScreen[_model.purchasedEngines[pe]._engineName];

(which is being looped through)

I errrr aaahhhh ummmmmm…???