Spring MVC problem: "model must not be null"

[sina poores]

hi to all friends

when i add to model a attribute in controller
@RequestMapping("/{username}")
	public String showWriterBlog(@PathVariable String username,Model model)
	{
		if(!wdao.isThereWriterWithThisUsername(username))
		{
			return "userBlog/blogNotFound";
		}
		else
		{
			System.out.println("model is: "+model);
			Writer writer=wdao.getWriterByUserName(username);
			model.addAttribute(writer);
			return "userBlog/blogHome";
		}
	}

i give this exception when i want to view distension page :

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.IllegalArgumentException: Model object must not be null
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:863)

[Mark Spritzler]

Which line exactly is giving that error. Is it hitting the if statement which returns true therefore just returns the string and never uses the Model object.

What happens if you set a break point on the first line of your controller? Does it get called? what is in the model if you do get to that line. Examine the variable in debug mode in your IDE.

Thanks

Mark