null pointer exception

The following program when run throws a null pointer exception from the 
call to MethodGen.getMethod (the stac trace says it is 
LocalVariableGen.getLocalVariable which is called by 
MethodGen.getLocalVariableaTable). Can anyone help?

clazz = new ClassGen("Program", "java.lang.Object", "<generated>",
                 Constants.ACC_PUBLIC | USE_INVOKESPECIAL_SEMANTICS,
                 IMPLEMENTS_NO_INTERFACES);
                 
constantPool = clazz.getConstantPool();
instructions = new InstructionList();
           
mainMethod = new MethodGen(Constants.ACC_STATIC | Constants.ACC_PUBLIC,
                                       Type.VOID,
                                       new Type[]{new 
ArrayType(Type.STRING, ONE_DIMENSIONAL)},
                                       new String[] {"args"},
                                       "main", "Program",
                                       instructions, constantPool);
                                      
factory = new InstructionFactory(clazz);

Instruction i = factory.createFieldAccess("java.lang.System", "out", new 
ObjectType("java.io.PrintStream"),
                Constants.GETSTATIC);
instructions.append(i);

instructions.append(new PUSH(constantPool, "Hello");

instructions.append(factory.createInvoke("java.io.PrintStream", "print", 
Type.VOID,
                new Type[]{Type.STRING}, Constants.INVOKEVIRTUAL));

instructions.append(InstructionConstants.RETURN);

mainMethod.setMaxStack(5);
clazz.addMethod(mainMethod.getMethod());
instructions.dispose();
clazz.addEmptyConstructor(Constants.ACC_PUBLIC);
clazz.getJavaClass().dump("Program.class");