Posting JSON data with Groovy's HTTPBuilder

I've found this doc on how to post JSON data using HttpBuilder. I'm new to this, but it is very straightforward example and easy to follow. Here is the code, assuming I had imported all required dependencies.

def http = new HTTPBuilder( 'http://example.com/handler.php' )
http.request( POST, JSON ) { req ->
    body = [name:'bob', title:'construction worker']

     response.success = { resp, json ->
        // response handling here
    }
}

Now my problem is, I'm getting an exception of

java.lang.NullPointerException
    at groovyx.net.http.HTTPBuilder$RequestConfigDelegate.setBody(HTTPBuilder.java:1131)

Did I miss something? I'll greatly appreciate any help you can do.