Discussed here: https://groups.google.com/d/msg/clojure-core-typed/njLzL8NinCc/7K9vxTZn__AJ
In both of the following cases core.typed'd thinks the HVec can contain nil, but it can't.
My original example:
Simpler example that might be the same thing (from Ambrose):
WIP here https://github.com/typedclojure/core.typed/pull/3
Not going to fix this myself, someone else can.