seeking help with error message

[im]

Hello,

I am getting an error message when executing this code (through a JSP page):

        Interpreter intr = new TreeInterpreter(new JavaCCParserFactory());
        Object x;
        String s = "a = b*2.0;";
        intr.defineVariable("b", new Double(10.0));
        intr.interpret(new StringReader(s), "topLevel");
        x = intr.getVariable("a");
        out.println(x)

The message is:
javax.servlet.ServletException: L1, C5 (topLevel):
Bad type in multiplication

Can someone point out to me what I am doing wrong?

Thanks in advance.