Nashorn bug retrieving array property after key string concatenation

FULL PRODUCT VERSION :
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Windows 7 x64

A DESCRIPTION OF THE PROBLEM :
Nashorn returns 'null' when externally binded array is called with key concatenated from several strings






ADDITIONAL REGRESSION INFORMATION:
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run the script above

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
array value returned
ACTUAL -
null returned whithout actually calling getMember

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
Java: VarsAdapter is a simple AbstractJSObject binded to the javascript context as "vars"

public class VarsAdapter extends AbstractJSObject
{
    @Override
    public Object getMember(String name)
    {
        return name;
    }
}

javascript:
var o = 6;
print( '1: price_'+o);
print( '2: '+vars['price_'+o]);
print( '3: '+vars['price_6']);
print( '4: '+vars[('price_'+o).toString()]);

output:

1: price_6
2: null
3: price_6
4: price_6

line 2 is incorrect. Debugger shows that getMember is never invoked when concatenated string is used as key.

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
use .toString() after string concatenation