ItemId and primarykey

Hi everybody;
I'm trying to find out of, why my itemid on inventtable isn't a primarykey. It's extending from the edt ItemId, and on that edt, there is a relation set up to inventTable.itemid.
If I'm using
fId = dt.primaryKeyField();

where fID is a fieldId at dt is a new DictTable(tablenum(inventTable)), it returns the value zero (0). There isn't any further modifications on the InventTable. I've also tried to set up a relation directly on the inventTable, with no
better result.
Hope some of you can give me a clue. I've also tried searching the Internet :-)

Best regards
Michael Hentrich