Nominatim problem postcode not returned

The request:
http://nominatim.openstreetmap.org/reverse?format=xml&lat=45.41660&lon=5.33245&addressdetails=1

return for the addressparts:
<addressparts>
<road>Route de Lyon</road>
<city>Mottier</city>
<county>Vienne</county>
<state>Rhône-Alpes</state>
<country>France</country>
<country_code>fr</country_code>
</addressparts>

But the tag <postcode> is not return

while the postal_code seems to be correctly set for the place Mottier
<node id="34055235" version="5" changeset="7048448" lat="45.4197" lon="5.31648" user="philippekerla" uid="79386" visible="true" timestamp="2011-01-22T10:45:43Z">
<tag k="code_departement" v="38"/>
<tag k="is_in" v="Europe,France,Isère"/>
<tag k="name" v="Mottier"/>
<tag k="place" v="village"/>
<tag k="population" v="500"/>
<tag k="postal_code" v="38260"/>
<tag k="ref:INSEE" v="38267"/>
<tag k="ref:SIREN" v="213802671"/>
</node>

Why ?
Regards Michel