This also works:
Bizarrely enough, even this works:
This produces a ClassCastException:
Likewise, this produces a similar ClassCastException:
There doesn't seem to be any documentation on into that implies it only works when kv pairs are represented as vectors (or somehow, maps), so this seems to be a bug. It's extremely surprising that it doesn't work for pairs represented as lists.
For the interested, I found this by writing a function to invert a map in the most natural way I could think of:
Confirmed with Clojure 1.7 on Ubuntu 3.13.0-63-generic 64bit.