AbstractMethodError trying to deftype with Object

I get this obscure message trying to upgrade timbre to cljs 1.7.228 (from 0.0.2727) and I have no idea what's changed.

java.lang.AbstractMethodError: Method cljsbuild/compiler/SourcePaths._find_sources(Ljava/lang/ObjectLjava/lang/Object; is abstract, compiling/private/var/folders/k6/glc05svd5d194k8kgqfwkrph0000gn/T/form-init382463497803898384.clj:1:124)

Code that triggers it is here:
https://github.com/jjl/automat/blob/cljc/src/automat/fsm.cljc#L25

I've eliminated my cljc as being the cause by bumping the clojurescript version on the original code and seeing the same error.