If I add "1.85:1" to the custom aspect ratio list and restart, I can cycle-switch to it using the "a" key and following the on-screen display, but when it comes to "1.85:1" the picture doesn't actually change. If I change it to "185:100" - which is basically the same - it behaves.
Possibly, other custom (but common) aspect ratios might also be affected.
[Running v2.1.5 on Windows 8]