RuntimeException "can't write to function" that only occurs on second execution using compiled expression but never using veal

evaluate("xpath('/root').attributes['newAttr']=1", message);
evaluate("xpath('/root').attributes['newAttr']=1", message);

The first time around this expression updates the DOM4J node returned by my xpath() function correctly. The second time it fails with a "can't write to function" RuntimeException. If I use MVEL.eval() that this doesn't occur. Also if i disable my compiled expression cache then things work too.

The root cause from what I can see is that the CompiledExpression is being mutated the on the first execution and this then affects the behavior of any further execution.

I've found the place where this happens:

org.mvel2.compiler.CompiledAccExpression.setValue(Object, Object, VariableResolverFactory, Object)

While getThreadAccessorOptimizer().optimizeSetAccessor(context, expression, 0, expression.length, ctx, ctx, vrf, false, value, ingressType); does exactly what it needed, the Accessor returned which is then in affect cached in the CompiledAccExpression instance does not and tries assigning '1' to the function directly thus failing.

It seems strange or maybe even incorrect that CompiledExpression is being mutated, especially if it changes it's behavior on subsequent executions.