QGIS Polygonize (GDAL) problem – Null value in attribute

Is it possible to use Polygonize in QGIS (GDAL) so that a line shapefile is converted to a polygon shapefile with an attribute(s) in the line file carried into the polygon file. I am trying to build an elevation polygon dataset – I downloaded data from EU-DEM, clipped and merged a few files into a new raster. Then created contours from the new raster. Now I want to create polygons – however the elevation value is not retained in the output polygon shapefile.
I have said yes to the option Keep Table Structure of Line Layer.
THe struture of the polygons are all fine but the value in the elevation attribute is NULL.
One way around of this is to do it for each value of elevation 100, 200….up to the max. Then add an attribute and set it to the correct value – however it would be quicker if Polygonize could do it. Am I missing something here? Is this a bug in Polygonize or was it just not designed to do this? Anyone got any ideas? I am using QGIS 2.4.