Magento WSI SOAP v2 API Custom Webservice Returns Empty Response -

problem: succesfully call webservice, empty response in return.

situation:

• i created new module structure , files following tutorial: magento: extending api (v2)
  • mynamespace
    • mymodule
      • helper
        • data.php
      • model
        • mymodel
          • api
            • v2.php
        • etc
          • api.xml
          • config.xml
          • wsdl.xml
          • wsi.xml

added module config in: app/etc/modules/mynamespace_mymodule.xml

• i debug call xdebug, , stops @ breakpoint inside webservice function
• the webservice supposed return simple string: "hello world"
• testing soapui empty response content length 0.

what can wrong or missing!? appreciated.

edit:

i've managed debug call , realized when reaches class zend_soap_server on line 832:

$soap->handle($request);

it doesn't execute further! don't know why.

the wsdl.xml had small mis-configuration in following line:

<binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/> 

it should be:

<soap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/>