[web2py] Cache.disk() expiring unexpectedly

[Dane]

I'm trying to use cache.disk to cache a large TABLE object
indefinitely. It seems to work fine at first, but then expires after a
couple minutes.

I'm setting it like this:

cache.disk.clear('^%s$' % cache_key)
cache.disk(cache_key, lambda:grid, None)

*(I also tried 10**10 instead of None for duration with same result)

And I'm accessing like this:

cached = cache.disk(cache_key, lambda:None)
if cached: return cached

-Dane