Why do I get a ClassCastException when parsing a tree?

Exception in thread "main" java.lang.ClassCastException:
org.antlr.runtime.tree.CommonTree cannot be cast to MyCustomASTNode

        nodes.setTreeAdaptor(astAdaptor);

    /** An adaptor that tells ANTLR to build CymbalAST nodes */
    public static TreeAdaptor astAdaptor = new CommonTreeAdaptor() {
        public Object create(Token token) { return new GrammarAST(token); }
        public Object dupNode(Object t) {
            if ( t==null ) return null;
            return create(((GrammarAST)t).token);
        }
        public Object errorNode(TokenStream input, Token start, Token stop,
                                RecognitionException e)
        {
            return new GrammarASTErrorNode(input, start, stop, e);
        }
    };

        BufferedTreeNodeStream nodes = new BufferedTreeNodeStream(astAdaptor,myRootNode);