stylus task yields spray.json.DeserializationException

[curreli]

I'm using Play 2.5.7 with sbt-stylus 1.0.0 running on the Node JsEngine. When I try to compile stylus source files, I get the following Exception:

[info] Stylus compiling on 1 source(s)
spray.json.DeserializationException: Expected Int as JsNumber, but got null
        at spray.json.package$.deserializationError(package.scala:23)
        at spray.json.ProductFormats$class.fromField(ProductFormats.scala:62)
        at com.typesafe.sbt.jse.SbtJsTask$JsTaskProtocol$.fromField(SbtJsTask.scala:92)
        at spray.json.ProductFormatsInstances$$anon$2.read(ProductFormatsInstances.scala:56)
        at spray.json.ProductFormatsInstances$$anon$2.read(ProductFormatsInstances.scala:46)
        at spray.json.JsValue.convertTo(JsValue.scala:31)
        at com.typesafe.sbt.jse.SbtJsTask$$anonfun$com$typesafe$sbt$jse$SbtJsTask$$executeSourceFilesJs$1$$anonfun$7.apply(SbtJsTask.scala:225)
        at com.typesafe.sbt.jse.SbtJsTask$$anonfun$com$typesafe$sbt$jse$SbtJsTask$$executeSourceFilesJs$1$$anonfun$7.apply(SbtJsTask.scala:224)
        at scala.collection.LinearSeqOptimized$class.foldLeft(LinearSeqOptimized.scala:111)
        at scala.collection.immutable.List.foldLeft(List.scala:84)
        at com.typesafe.sbt.jse.SbtJsTask$$anonfun$com$typesafe$sbt$jse$SbtJsTask$$executeSourceFilesJs$1.apply(SbtJsTask.scala:223)
        at com.typesafe.sbt.jse.SbtJsTask$$anonfun$com$typesafe$sbt$jse$SbtJsTask$$executeSourceFilesJs$1.apply(SbtJsTask.scala:221)
        at scala.util.Success$$anonfun$map$1.apply(Try.scala:206)
        at scala.util.Try$.apply(Try.scala:161)
        at scala.util.Success.map(Try.scala:206)
        at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235)
        at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:235)
        at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
        at scala.concurrent.impl.ExecutionContextImpl$$anon$3.exec(ExecutionContextImpl.scala:107)
        at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
        at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.pollAndExecAll(ForkJoinPool.java:1253)
        at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1346)
        at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
        at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
[error] (frontend/web-assets:stylus) spray.json.DeserializationException: Expected Int as JsNumber, but got null
[error] Total time: 1 s, completed 13 sept. 2016 23:52:17

I've seen there has been similar issues with sbt-less (sbt/sbt-less#38, sbt/sbt-less#41) when compiling empty files. I have no empty files and it still fails.

Any clue?

UPDATE: looks like this happens only on Windows