Why do I get java.lang.NoClassDefFoundError when I trying to run this code?

I want to map over the characters in a string, but I'm getting runtime errors.

Example:

object Hello {
    def hello(c: Char) {
        print(c)
    }

    def main(args: Array[String]) {
        "Hello World!".map(hello)
    }
}

Trace:

scalac Hello.scala
java Hello
Exception in thread "main" java.lang.NoClassDefFoundError: scala/LowPriorityImplicits
    at Hello.main(Hello.scala)
Caused by: java.lang.ClassNotFoundException: scala.LowPriorityImplicits
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    ... 1 more
make: *** [test] Error 1