jjs (the Nashorn JavaScript tool) prints erroneous line "null"

FULL PRODUCT VERSION :
java version "1.8.0_31"
Java(TM) SE Runtime Environment (build 1.8.0_31-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.31-b07, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.3.9600]

EXTRA RELEVANT SYSTEM CONFIGURATION :
output of jjs -version: nashorn 1.8.0_31


A DESCRIPTION OF THE PROBLEM :
Running the example from http://docs.oracle.com/javase/8/docs/technotes/guides/scripting/nashorn/api.html#sthref19 does not produce the expected result

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
on the command line, do:
>jjs
jjs> Java.type("java.lang.System").out.println(10)

then you'll get:

>jjs
jjs> Java.type("java.lang.System").out.println(10)
10
null
jjs>

the problem is that an erroneous line with "null" is printed.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
>jjs
jjs> Java.type("java.lang.System").out.println(10)
10
jjs>
ACTUAL -
>jjs
jjs> Java.type("java.lang.System").out.println(10)
10
null
jjs>

REPRODUCIBILITY :
This bug can be reproduced always.