[MapProxy] invalid wms request from mapproxy

[Travis Kirstine]

I noticed a bunch of 500 errors being generated in MapProxy because the
source  MapServer WMS is not generating images.  It appears some cases
MapProxy will generate a WMS request with a width=1 and height=3 px.  I
think this is related to the source coverage (bbox) in some way.  The
source has a fairly small bbox which could be the problem? If I remove the
coverage from the source wms it works

 /cgi-bin/mapserver_wms?styles=&format=image%2Fpng&height=1&bbox=-11496569.213,5478916.2662,-11494537.6069,5484007.27162&transparent=True&layers=BHP&service=WMS&width=1&request=GetMap&srs=EPSG%3A3857&version=1.1.1

The odd thing is MapProxy is working fine when looking at the layer between
zoom 19-7 however once I zoom out between 0-6 the request start to fail

MP Stuff:

  BHP_cache:
    grids: [spherical_mercator]
    sources: [BHP_wms]
    meta_buffer: 100
    meta_size: [4,4]
    format: image/png

  BHP_wms:
    type: wms
    req:
      url: http://localhost/cgi-bin/mapserver_wms?
      layers: BHP
      transparent: true
    coverage:
      bbox:
[-11496569.2130269,5478916.26620355,-11494537.6069391,5484007.27161516]
      srs: 'EPSG:3857'
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