ST_CreateOverview Function Generating Overview-Rasters With Black Grids

Hi All,

I used the ST_CreateOverview function to create overviews for a sample
raster (256 x 256 pixels; 14,400 rows) as follows:

SELECT ST_CreateOverview('<raster_table>'::regclass,'rast_column'::name,
<factor>);
where <factor> used is 2,3,4,5,6,7,8

When I try to view the raster in QGIS I notice some black grids appear at
higher-levels of zoom which would obviously be from the overview table (*see
attached image*), but there are no black grids at lower-levels of zoom
which would obviously be from the actual raster table (*see attached image*
).

I can see that someone complained of a similar problem for the same
function in the link below, but there was no solution:
http://osgeo-org.1560.x6.nabble.com/st-createoverview-td5261112.html

Could someone explain the likely cause for this and how to avoid having the
black grids from the overviews.