[antlr-interest] keep getting java.lang.NullPointerException using xmlSerialize()

[Henk-Jan Ebbers]

using antlr 2.7.5, java 1.5.0_01

I want output parsed file as XML

            FileInputStream test = new FileInputStream("test.edi");       
            EdifactLexer lexer = new EdifactLexer(new 
DataInputStream(test));
            EdifactParser parser = new EdifactParser(lexer);
            parser.start();
            antlr.BaseAST resultTree=(antlr.BaseAST)parser.getAST();
            Writer w = new OutputStreamWriter(System.out);
            resultTree.xmlSerialize(w);
            w.write("\n");
            w.flush();

large part of the tree input-file is written to System.out,
but then:
java.lang.NullPointerException
        at antlr.BaseAST.encode(BaseAST.java:409)
        at antlr.BaseAST.xmlSerializeNode(BaseAST.java:452)
        at antlr.BaseAST.xmlSerialize(BaseAST.java:479)
        at Main.main(Main.java:23)

Am i missing something?

greetings,
Henk-Jan