'runtime error using akka http -> java.lang.ClassNotFoundException: akka.stream.Attributes$CancellationStrategy$Strategy

I'm getting this error trying to setup a sample akka http integration in my existing akka stream integration.

package ui

import akka.actor.ActorSystem
import akka.stream.ActorMaterializer
import akka.http.scaladsl.Http
import akka.http.scaladsl.model.HttpMethods._
import akka.http.scaladsl.model._
import scala.concurrent.ExecutionContext
import scala.io.StdIn

object Main extends App {

    implicit val system = ActorSystem("lowlevel")
    // needed for the future map/flatmap in the end
    implicit val executionContext: ExecutionContext = system.dispatcher
    implicit val materializer: ActorMaterializer = ActorMaterializer()

    val requestHandler: HttpRequest => HttpResponse = {
      case HttpRequest(GET, Uri.Path("/"), _, _, _) =>
        HttpResponse(entity = HttpEntity(
          ContentTypes.`text/html(UTF-8)`,
          "<html><body>Hello world!</body></html>"))

      case HttpRequest(GET, Uri.Path("/ping"), _, _, _) =>
        HttpResponse(entity = "PONG!")

      case HttpRequest(GET, Uri.Path("/crash"), _, _, _) =>
        sys.error("BOOM!")

      case r: HttpRequest =>
        r.discardEntityBytes() // important to drain incoming HTTP Entity stream
        HttpResponse(404, entity = "Unknown resource!")
}

    val bindingFuture = Http().newServerAt("localhost", 8080).bindSync(requestHandler)
    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done


}

The code compiles fine, but when I run tests on the top level class that has the endpoint setup I get this runtime error:

java.lang.ClassNotFoundException: akka.stream.Attributes$CancellationStrategy$Strategy
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)

any idea what might be causing this runtime class not found issue?

I'm using "10.1.14" of akka http and "2.5.31" of the akka / akka-stream libraries on scala 2.12.15

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'runtime error using akka http -> java.lang.ClassNotFoundException: akka.stream.Attributes$CancellationStrategy$Strategy

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