Webservices Pass empty Value in modify method

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Hi there, 

I'm trying to "clear" a field using the method Modify in a PHP script with SOAP from another plataform, but it's not working, if the field is empty in the XML it appears that it doesn't modify.

I tried:

<PARAM>
<FLD NAME="WS2GRUPO"></FLD>
</PARAM>

AND

<PARAM>
<FLD NAME="WS2GRUPO"/>
</PARAM>

Neither one works, is there a way to achieve this using the method Modify?