Get InstanceObject's Sub-Objects

Im working with C#, and im trying to get an InstanceObjects' Objects.  When using a InstanceDefinition, I would use .GetObjects(), So I assume I should use .GetSubObjects() when working with InstanceObjects.  However when I use .GetSubObjects, I am not able to access the Guid of the SubObjects.  Does .GetObjects and .GetSubObjects not work the same way, just with different types?  Or is the error I am getting not related to this.  

For Reference, my code looks something like this:

Rhino.DocObject.RhinoObject[] myObjs;

myObjs = myInstanceObject.GetSubObjects();

int i = 0;

while(i < myObjs.Count())

{

Print(myObjs[i].Attributes.ObjectId.ToString());

i++;

}

And the error I am getting is this:

error: This object cannot be modified because it is controlled by a document. (line: 0)