"Content is not allowed in prolog" error when soapaction is removed in a WSDL with a SOAP Proxy ESB. How to fix it?

Issue

I have a WEB Service and an ESB Proxy for it. When removing the soapaction attribute in soap:operation element from the Web Service WSDL, the following exception is thrown:

    WARN  [org.jboss.soa.esb.actions.soap.proxy.SOAPProxy] (pool-70-thread-7) null binding for soapaction [""]; parsing envelope to find element or operation...
    ERROR [STDERR] (pool-70-thread-7) [Fatal Error] :2:1: Content is not allowed in prolog.
    ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/SoapProxy].[HttpGateway]] (http-16.181.236.106-8080-1) Servlet.service() for servlet HttpGateway threw exception
    org.xml.sax.SAXParseException: Content is not allowed in prolog.
            at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
            at org.jboss.soa.esb.actions.soap.proxy.SOAPProxy.findElement(SOAPProxy.java:392)
            at org.jboss.soa.esb.actions.soap.proxy.SOAPProxy.process(SOAPProxy.java:316)
            at org.jboss.soa.esb.listeners.message.ActionProcessingPipeline.processPipeline(ActionProcessingPipeline.java:635)
            at org.jboss.soa.esb.listeners.message.ActionProcessingPipeline.processPipeline(ActionProcessingPipeline.java:589)
            at org.jboss.soa.esb.listeners.message.ActionProcessingPipeline.process(ActionProcessingPipeline.java:419)
            at org.jboss.soa.esb.listeners.message.MessageAwareListener$TransactionalRunner.run(MessageAwareListener.java:540)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
            at java.lang.Thread.run(Thread.java:636)