Failing to see why cast does not work in GraalJS

Question

I am failing to see why this code fails.

$ C:/graalvm-jdk-20_windows-x64_bin/graalvm-jdk-20.0.2+9.1/bin/js.cmd --jvm --vm.Xss1g --vm.Xmx4g --jvm --vm.classpath="C:/Users/john/pythonSAI/X3DJSAIL.4.0.full.jar" ExJin.js


TypeError: execute on org.web3d.x3d.jsail.HAnim.HAnimHumanoid failed due to: Message not supported.
        at <js> :program(ExJin.js:1-2:0-39)

$ cat HAnimHumanoid.js
HAnimHumanoid = Java.type('org.web3d.x3d.jsail.HAnim.HAnimHumanoid')

$ cat ExJin.js
load('HAnimHumanoid.js')
(HAnimHumanoid)(new HAnimHumanoid()).setName("foo");

I will try removing the cast, but I think that may create other issues.