Cannot download large assetbundle with WWW.LoadFromCacheOrDownload on old iOS device

Cannot download large assetbundle (About 40MB) with WWW.LoadFromCacheOrDownload
even if we have enough space to download this

It just says “Not enough space in cache to write file”

And strangely Caching.maximumAvailableDiskSpace and Caching.spaceFree returns very large number
9223372036854775807 ( maybe max number for int64)

It happens only on iPhone 5 not on iPhone 6

Is there any workaround for this? for some reason we cannot divide our asset bundle to small bundle.
We are using unity 5.3.4p4

It seems I can set Caching.maximumAvailableDiskSpace value myself
so I set this value to 1GB
Caching.maximumAvailableDiskSpace = 1024 * 1024 * 1024;

but I get same result.

I can’t download 40MB asset bundle.