[MapProxy] Seed zoom level 20 (version 1.6)

[Billy Newman]

I am using version 1.6 I am trying to seed up to level 20:

seeds:
 test_seed:
   caches: [test_cache]
   coverages: [test_coverage]
   levels:
     from: 0
     to: 20

And it will only seed to level 19.

If I try 20-20:
seeds:
 test_seed:
   caches: [test_cache]
   coverages: [test_coverage]
   levels:
     from: 20
     to: 20

When I run I get the following error:
========== Seeding tasks ==========
Start seeding process (1 task)
 test_seed:
   Seeding cache 'test_cache' with grid 'webmercator_nw' in EPSG:3857
   Levels: []
   Overwriting: no tiles
Traceback (most recent call last):
 File "/usr/local/bin/mapproxy-seed", line 9, in <module>
   load_entry_point('MapProxy==1.6.0', 'console_scripts', 'mapproxy-seed')()
 File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mapproxy/seed/script.py", line 234, in main
   return SeedScript()()
 File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mapproxy/seed/script.py", line 165, in __call__
   skip_geoms_for_last_levels=options.geom_levels)
 File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mapproxy/seed/seeder.py", line 412, in seed
   seed_progress=seed_progress)
 File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mapproxy/seed/seeder.py", line 429, in seed_task
   seed_progress=seed_progress)
 File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mapproxy/seed/seeder.py", line 234, in __init__
   self.report_till_level = task.levels[int(num_seed_levels * 0.8)]
IndexError: list index out of range


I can seed all other zoom levels, is there a hard limit?

Thanks in advance.