Nashorn should return "undefined" when looking for properties that don't exist

A DESCRIPTION OF THE PROBLEM :
#Testcase:
1. var NISLFuzzingFunc = function (placements) {
2. var p = undefined;
3. print(placements[p]);
4. };
5. var NISLParameter1 = [9814390527, 2];
6. NISLFuzzingFunc(NISLParameter1);

#Command:
./nashorn/jdk-12.0.2/bin/jjs --no-syntax-extensions=true --no-deprecation-warning=true testcase.js

#Output:
NaN

#Expected output:
undefined

#Description:
If the specified property can not be found, Nashorn should return "undefined" instead of "NaN". However, the output of Nashorn is "NaN" when running the testcase above.
Is this a bug of Nashorn? Looking forward to your reply.