UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/50.0.2661.94 Safari/537.36

Steps to reproduce the problem:
1. Register a service worker
2. Setup a write-through cache based on any of the online guides
3. Add .then(res => { console.log(res); return res }) as a final operation in the promise chain, after the catch

What is the expected behavior?
A resource is always returned

What went wrong?
Examples that use 'catch' work most of the time, but occasionally return null. Examples that use if(!res) fail with DOMExceptions.

Did this work before? N/A 

Chrome version: 50.0.2661.94  Channel: stable
OS Version: OS X 10.11.3
Flash Version: Shockwave Flash 21.0 r0

It seems to only be possible to make this stable by doing:

caches.match(req).then((res) => { if (!res) throw 'error'; return res }).catch(... write-through logic ...)

this feels messy.

I read the spec and it looks like caches.match should always error on missing items, so I'm not clear on how empty responses are returned - they don't appear in my cache.

This does not happen reliably for all files, but instead appears to occur for particular items in a repetitive way, (that cause either a null or a domexception but not both), but not consistently for file types or origins.