I have the following war task in my build.xml and even though needxmlfile is set to false, Ant (version 1.8.2) complains when the web.xml file does not exist ("BUILD FAILED ... Deployment descriptor: /home/.../web/WEB-INF/web.xml does not exist")

What am I missing?

  <target name="war" depends="build">                                                                           
    <mkdir dir="${build.dir}"/>                                                                                 
    <war                                                                                                        
      needxmlfile="false"                                                                                       
      basedir="${webroot.dir}"                                                                                  
      warfile="${build.dir}/${project.distname}.war"                                                            
      webxml="${webinf.dir}/web.xml">                                                                           
      <exclude name="WEB-INF/${build.dir}/**"/>                                                                 
      <exclude name="WEB-INF/src/**"/>                                                                          
      <exclude name="WEB-INF/web.xml"/>                                                                         
     </war>                                                                                                     
  </target>