DTDException: Undefined Symbol thrown

I have a soap web service client - server application. Wanted to test Billion laugh attack in the soap request. But regardless of XML bomb, even very simple DTD like the below gives the following exception before reaching the server code.

DTD:

    <?xml version="1.0"?>
    <!DOCTYPE Hi[
    <!ENTITY HelloNode "SayHi">
    ]>

Usage:

       <!--Some Nodes-->
       <Hi>&HelloNode;</Hi>
       <!--Some Nodes-->

Exception:

    -----------------TABLE REF---------------

   weblogic.xml.babel.dtd.DTDException: Undefined Symbol:HelloNode
   Error at line:11 col:26  Unknown entity reference: 'HelloNode' 
      at weblogic.xml.babel.scanner.Reference.read(Reference.java:175)

I use SoapUI to trigger the request. There is no issue with XML as it's confirmed by successful validation by the SoapUI. Any clues on why this exception is thrown?