RegExp implementation is not thread-safe

The following script:

var Thread = java.lang.Thread;

function test_thread() {
  var line = 'content-type: text/html';
  while (1) {
    Thread.sleep(100);
    line.split(/: /);
  }
}

for (var i=0; i<5; i++) {
  new Thread(test_thread).start();
}

while (1) { Thread.sleep(1000); }

Throws:

java.lang.NullPointerException
at jdk.nashorn.internal.runtime.regexp.joni.Regex.matcher(Regex.java:148)
at jdk.nashorn.internal.runtime.regexp.joni.Regex.matcher(Regex.java:143)
at jdk.nashorn.internal.runtime.regexp.JoniRegExp$JoniMatcher.<init>(JoniRegExp.java:126)
at jdk.nashorn.internal.runtime.regexp.JoniRegExp.match(JoniRegExp.java:101)
at jdk.nashorn.internal.objects.NativeRegExp.execSplit(NativeRegExp.java:581)
at jdk.nashorn.internal.objects.NativeRegExp.split(NativeRegExp.java:824)
at jdk.nashorn.internal.objects.NativeString.split(NativeString.java:878)