[MapProxy] Layer becomes black

[Denis Rykov]

Faced with the very strange issue.

I have the following configuration: source as MapServer WMS (TIF
raster) -> MapProxy -> WMS Service.
If MaProxy requests extent which not contains any data then I get
black image, otherwise all works as expected.

You can check live demo here: http://rykovd.nextgis.com/resource/358/display

MapProxy config:

caches:
  bfa_cache:
    grids: [webmercator]
    sources: ['mapserver:bfa']
    cache:
      type: sqlite

sources:
  mapserver:
    type: mapserver
    wms_opts:
      featureinfo: true
    req:
      map: /opt/mapserver/mapfile.map
      transparent: true

Any thoughts?