Set union weirdness

[kwatford]

So I have some code that looks somewhat like:

(update-in node [:index :x] set/union #{:y})

This seems fine as long as there's already a set in the desired spot.
However, since update-in will kindly build the map structure for you
if it doesn't already exist, I figured it would be nice not to have to
check for nils and replace them with empty sets. So I tried this:

=> (set/union nil #{:foo})
(:foo)

Huh? I checked its class, and it is indeed a list. So I tried:
=> (set/union '(1 2 3 a) '(a b c))
(c b a 1 2 3 a)

=> (set/union [0 1 2 3] [0])
[0 1 2 3 0]

Looking at the source explains this:
(defn union
"Returns a set that is the union of the two sets."
[xset yset]
(reduce conj xset yset))

I don't know if this (and any similar weirdness in the rest of the set
namespace) should be fixed since you aren't really supposed to throw
non-sets in there, but I do think a special case for nil might be nice
here.