QGIS » QGIS Application

I have a polygon shapefile with an attribute villageID, in which some polygons have the same attribute value and so they should together form a single multi-part polygon. Some other polygons have NULL in this villageID field. When I run the tool Vector>Geometry Tools>Singleparts to Multipart (from the menu or from Procsesing Toolbox, and whether I use the GDAL version or the QGIS geoalgorithm version), the resulting shapefile does merge the repeated villageIDs into multi-part polygons. But it ALSO merges ALL the polygons that had this field = NULL into a SINGLE multi-part polygon. In effect, it is treating all NULL values as equal, when in fact it should treat NULL as 'not known' and ignore those polygons. If I run a 'find duplicates' query in MS-ACCESS on the attribute table dbf, the query ignores those rows where villageID=NULL, which IMHO is the correct method.