spray.json.SerializationException: Map key must be formatted as JsString, not '7375'

[zhenghub]

I have a Map[Int, Int] type value called deltaTime. And when I "import DefaultJsonProtocol._" and invoke "deltaTime.toJson", spray.json tell me an exception as below. Does spray.json require the key of Map must be of String type?

Exception in thread "main" spray.json.SerializationException: Map key must be formatted as JsString, not '7375'
at spray.json.CollectionFormats$$anon$3$$anonfun$write$3.apply(CollectionFormats.scala:53)
at spray.json.CollectionFormats$$anon$3$$anonfun$write$3.apply(CollectionFormats.scala:50)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.immutable.HashMap$HashMap1.foreach(HashMap.scala:224)
at scala.collection.immutable.HashMap$HashTrieMap.foreach(HashMap.scala:403)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
at scala.collection.AbstractTraversable.map(Traversable.scala:105)
at spray.json.CollectionFormats$$anon$3.write(CollectionFormats.scala:50)
at spray.json.CollectionFormats$$anon$3.write(CollectionFormats.scala:48)
at spray.json.PimpedAny.toJson(package.scala:39)

[wlk]

This is a duplicate of: #125

[ktoso]

Closing as duplicate