Stack trace of possible MapProxy issue (MapError)

Stack trace

Process TileSeedWorker-1:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/seed/seeder.py", line 92, in run
    self.work_loop()
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/seed/seeder.py", line 104, in work_loop
    exceptions=(SourceError, IOError), ignore_exceptions=(LockTimeout, ))
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/seed/util.py", line 216, in exp_backoff
    result = func(*args, **kw)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/cache/tile.py", line 126, in load_tile_coords
    created_tiles = creator.create_tiles(uncached_tiles)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/cache/tile.py", line 213, in create_tiles
    created_tiles = self._create_single_tiles(tiles)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/cache/tile.py", line 237, in _create_single_tiles
    created_tiles.extend(self._create_single_tile(tile))
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/cache/tile.py", line 253, in _create_single_tile
    source = self._query_sources(query)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/cache/tile.py", line 274, in _query_sources
    return self.sources[0].get_map(query)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/source/tile.py", line 95, in get_map
    return CacheMapLayer.get_map(self, query)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/layer.py", line 390, in get_map
    self._check_tiled(query)
  File "/home/steven/Virtualenv/workingMapproxy/lib/python2.7/site-packages/mapproxy/layer.py", line 409, in _check_tiled
    raise MapError("invalid tile format, use %s" % self.tile_manager.format)
MapError: invalid tile format, use png