Reagent Figwheel npes during build phase - clojure

Question

I'm running lein figwheel + reagent, and while the the site seems to be updating, I can't track down what would cause the following stacktrace. Is there anyplace I should look? What does this error mean?
lein figwheel
Figwheel: focusing on build-id 'app'
Compiling ClojureScript.
Figwheel: Starting server at http://localhost:3449
Figwheel: Serving files from '(dev-resources|resources)/public'
Compiling "resources/public/js/app.js" from ("src/cljs" "env/dev/cljs")...
Compiling "resources/public/js/app.js" failed.
java.lang.NullPointerException:
closure.clj:325 cljs.closure/compiled-file
core.clj:2557 clojure.core/map[fn]
LazySeq.java:40 clojure.lang.LazySeq.sval
LazySeq.java:49 clojure.lang.LazySeq.seq
RT.java:484 clojure.lang.RT.seq
core.clj:133 clojure.core/seq
core.clj:685 clojure.core/concat[fn]
LazySeq.java:40 clojure.lang.LazySeq.sval
LazySeq.java:49 clojure.lang.LazySeq.seq
RT.java:484 clojure.lang.RT.seq
core.clj:133 clojure.core/seq
core.clj:685 clojure.core/concat[fn]
LazySeq.java:40 clojure.lang.LazySeq.sval
LazySeq.java:49 clojure.lang.LazySeq.seq
Cons.java:39 clojure.lang.Cons.next
RT.java:1654 clojure.lang.RT.boundedLength
RestFn.java:130 clojure.lang.RestFn.applyTo
core.clj:626 clojure.core/apply
closure.clj:993 cljs.closure/build
core.clj:170 clojurescript-build.core/build-source-paths
auto_builder.clj:59 figwheel-sidecar.auto-builder/autobuild[fn]
auto_builder.clj:52 figwheel-sidecar.auto-builder/autobuild
form-init495530074882707163.clj:1 user/eval12375
Compiler.java:6703 clojure.lang.Compiler.eval
Compiler.java:6693 clojure.lang.Compiler.eval
Compiler.java:7130 clojure.lang.Compiler.load
Compiler.java:7086 clojure.lang.Compiler.loadFile
main.clj:274 clojure.main/load-script
main.clj:279 clojure.main/init-opt
main.clj:307 clojure.main/initialize
main.clj:342 clojure.main/null-opt
main.clj:420 clojure.main/main
RestFn.java:421 clojure.lang.RestFn.invoke
Var.java:383 clojure.lang.Var.invoke
AFn.java:156 clojure.lang.AFn.applyToHelper
Var.java:700 clojure.lang.Var.applyTo
main.java:37 clojure.main.main
Here's my project.clj:
(defproject project-name "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:url "http://example.com/FIXME"
:license {:name "Eclipse Public License"
:url "http://www.eclipse.org/legal/epl-v10.html"}
:source-paths ["src/clj" "src/cljs"]
:dependencies [[org.clojure/clojure "1.6.0"]
[com.facebook/react "0.11.2"]
[reagent "0.5.0-alpha"]
[reagent-utils "0.1.0"]
[secretary "1.2.1"]
[org.clojure/clojurescript "0.0-2496" :scope "provided"]
[com.cemerick/piggieback "0.1.3"]
[weasel "0.4.2"]
[ring "1.3.2"]
[ring/ring-defaults "0.1.2"]
[prone "0.8.0"]
[compojure "1.3.1"]
[selmer "0.7.7"]
[environ "1.0.0"]
[leiningen "2.5.0"]
[figwheel "0.1.5-SNAPSHOT"]
[cljs-ajax "0.3.3"]]
:plugins [[cider/cider-nrepl "0.8.2"]
[lein-cljsbuild "1.0.3"]
[lein-environ "1.0.0"]
[lein-ring "0.8.13"]
[lein-asset-minifier "0.2.0"]]
:ring {:handler homing-pigeon.handler/app}
:min-lein-version "2.5.0"
:uberjar-name "homing-pigeon.jar"
:minify-assets
{:assets
{"resources/public/css/site.min.css" "resources/public/css/site.css"}}
:cljsbuild {:builds {:app {:source-paths ["src/cljs"]
:compiler {:output-to "resources/public/js/app.js"
:output-dir "resources/public/js/out"
:externs ["react/externs/react.js"]
:optimizations :none
:pretty-print true}}}}
:profiles {:dev {:repl-options {:init-ns homing-pigeon.handler
:nrepl-middleware [cemerick.piggieback/wrap-cljs-repl]}
:dependencies [[ring-mock "0.1.5"]
[ring/ring-devel "1.3.2"]
[pjstadig/humane-test-output "0.6.0"]]
:plugins [[lein-figwheel "0.2.0-SNAPSHOT"]]
:injections [(require 'pjstadig.humane-test-output)
(pjstadig.humane-test-output/activate!)]
:figwheel {:http-server-root "public"
:server-port 3449
:css-dirs ["resources/public/css"]
:ring-handler homing-pigeon.handler/app}
:env {:dev? true}
:cljsbuild {:builds {:app {:source-paths ["env/dev/cljs"]
:compiler {:source-map true}}}}}
:uberjar {:hooks [leiningen.cljsbuild minify-assets.plugin/hooks]
:env {:production true}
:aot :all
:omit-source true
;;TODO: figure out how to clean properly
;:prep-tasks [["cljsbuild" "clean"]]
:cljsbuild {:jar true
:builds {:app
{:source-paths ["env/prod/cljs"]
:compiler
{:optimizations :advanced
:pretty-print false}}}}}
:production {:ring {:open-browser? false
:stacktraces? false
:auto-reload? false}}})

Answer 1

I found the problem.
I had an empty file in core/rules.cljs, and the compiler was erorring out over it.