I used zonal statistics through QGIS to assign raster image values to polygons.
Here, Raster image has value in range from 0 to 238.567.
As a result, some NULL values are assigned to polygons,
even though raster image fully covers those polygons.
When "count" is assigned to zero, statistical values are set to NULL. But such a polygon is still on the raster image(at least visually).
What's wrong with it ?