adding LayerTableRecord

Hi,
I have this code to create a new layer:

            LayerTable lt = (LayerTable)tr.GetObject(db.LayerTableId, OpenMode.ForWrite);

            if (lt.Has(layerName))
                return;

            LayerTableRecord x = new LayerTableRecord();

            x.Name = layerName;

            x.LineWeight = LineWeight.ByLineWeightDefault;
            x.IsOff = false;
            x.IsFrozen = false;
            x.IsLocked = false;
            x.Color = c;
            x.IsPlottable = true;

            x.Description = layerDescription;

            lt.Add(x);
            tr.AddNewlyCreatedDBObject(x, true);

When the line
x.Description = layerDescription;
is executed I have an eNotDatabase exception.
This sounds very strange for me and I don't understand where the problem is.

Thank you very much