I would get the error also, but you can do it this way.
[CommandMethod( "TestTableToBlock" )]
public void TestTableToBlock() {
Document Doc = acadApp.DocumentManager.MdiActiveDocument;
Database Db = Doc.Database;
Editor Ed = Doc.Editor;
using ( Transaction Trans = Db.TransactionManager.StartTransaction() ) {
PromptEntityResult per = Ed.GetEntity( "\n Select table: " ) as PromptEntityResult;
Table tbl = Trans.GetObject( per.ObjectId, OpenMode.ForWrite ) as Table;
BlockTableRecord LoRec = Trans.GetObject( Db.CurrentSpaceId, OpenMode.ForWrite ) as BlockTableRecord;
BlockReference BlkRef = new BlockReference( tbl.Position, tbl.BlockTableRecord );
LoRec.AppendEntity( BlkRef );
Trans.AddNewlyCreatedDBObject( BlkRef, true );
tbl.Erase();
Trans.Commit();
}
}