Serving MBTiles file via MapProxy

I'm trying to serve an existent MBTiles file using MapProxy. My YAML file looks like this:

services:
  wms:
    srs: ['EPSG:3785', 'EPSG:4326', 'EPSG:23028']

    image_formats: ['image/jpeg', 'image/png']

layers:
  - name: my_mbtiles_layer
    title: My MBTiles layer
    sources: [my_mbtiles_cache]

cache:
  my_mbtiles_cache:
    sources: []

    grids: [my_grid]
    cache:
      type: mbtiles
      filename: /path/to/myfile.mbtiles

grids:
  my_grid:
    base: GLOBAL_GEODETIC
    res_factor: 'sqrt2'
    bbox_srs: 'EPSG:4326'

    bbox: [-6.54,38.35,-3,40.65]

So, I defined my own grid, based on GLOBAL_GEODETIC, and used it for my MBTiles cache, that is the only source for my layer.

And the queries look like these:

http://myserver/service?LAYERS=my_mbtiles_layer&FORMAT=image%2Fpng&SRS=EPSG%3A4326&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&BBOX=-6.54,38.35,-3,40.65&WIDTH=256&HEIGHT=256

All my tiles form a coverage inside the BBOX (in epsg:4326): [-6.54,38.35,-3,40.65]

But the problem is I get a blank image of 256x256 px, and a HTTP code 200. So, the response is ok, but nothing is rendered in it. The MBTiles coverage is ok. I can see it in a desktop viewer. Here, an screenshot:

http://dl.dropbox.com/u/6599273/capturas/mbtiles_coverage.png

So, where can be the problem?

Many thanks in advance

Answer

I finally solved it by generating the zoom levels from 0 in gdal2tiles and using GLOBAL_MERCATOR grid.