Unable to run Javascript in Neoload 6.6?

I've used javascript in Neoload before to write things to the log but am finding with 6.6 it causes execution to crash.
It'll execute a couple of times fine and then there's a stack dump in the log.
There's a stack dump in the log:
The first couple of lines are the correct output then the dump!
2018/11/13 16:12:27 DEBUG - neoload.JavaScript.writeLog: Invoice quni00002with 00002 lines has status Submitted
2018/11/13 16:12:30 DEBUG - neoload.JavaScript.writeLog: Invoice quni00004with 00004 lines has status Submitted
2018/11/13 16:12:34 FATAL - neoload.Misc: Error while calling evaluateScript method java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.neotys.nl.js.f.a(f.java:50)
at com.neotys.nl.lg.engine.Q.internalExecute(Q.java:61)
at com.neotys.nl.lg.engine.l.a(l.java:146)
at com.neotys.nl.lg.engine.v.c(v.java:125)
at com.neotys.nl.lg.engine.w.internalExecute(w.java:83)
at com.neotys.nl.lg.engine.l.a(l.java:146)
at com.neotys.nl.lg.engine.H.i(H.java:251)
at com.neotys.nl.lg.engine.H.internalExecute(H.java:98)
at com.neotys.nl.lg.engine.l.a(l.java:146)
at com.neotys.nl.lg.engine.c.a(c.java:177)
at com.neotys.nl.lg.engine.c.run(c.java:108)
Caused by: java.lang.IllegalAccessError: class sun.reflect.GeneratedMethodAccessor9 cannot access its superclass sun.reflect.MethodAccessorImpl
at sun.misc.Unsafe.defineClass(Native Method)
at sun.reflect.ClassDefiner.defineClass(ClassDefiner.java:63)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:399)
at sun.reflect.MethodAccessorGenerator$1.run(MethodAccessorGenerator.java:394)
at java.security.AccessController.doPrivileged(Native Method)
at sun.reflect.MethodAccessorGenerator.generate(MethodAccessorGenerator.java:393)
at sun.reflect.MethodAccessorGenerator.generateMethod(MethodAccessorGenerator.java:75)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:53)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.neotys.nl.js.c.a(c.java:38)
at com.neotys.nl.js.c.g(c.java:102)
at com.neotys.nl.js.api.i.e(i.java:90)
at com.neotys.nl.js.api.VariableManager.jsFunction_getValue(VariableManager.java:67)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.mozilla.javascript.MemberBox.invoke(MemberBox.java:161)
at org.mozilla.javascript.FunctionObject.call(FunctionObject.java:476)
at org.mozilla.javascript.optimizer.OptRuntime.call1(OptRuntime.java:66)
at org.mozilla.javascript.gen.writeLog_1._c_script_0(writeLog:11)
at org.mozilla.javascript.gen.writeLog_1.call(writeLog)
at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:426)
at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3178)
at org.mozilla.javascript.gen.writeLog_1.call(writeLog)
at org.mozilla.javascript.gen.writeLog_1.exec(writeLog)
at com.neotys.nl.js.api.d.evaluateScript(d.java:351)
... 15 more