NullPointerException when sending a struct in an any object

I'm trying to send a struct defined by an IDL. What I do is:

rg.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init();
Any val = orb.create_any();

Property prop= new Property();
prop.pName="pName";
prop.pValue="pValue";
PropertyHelper.insert(val, prop);

Parameter[] params = new Parameter[1];
params[0] = new Parameter("prop", val);

BOMonitoringSet boMonitoringSet = (BOMonitoringSet)
BOMonitoringSetHelper.narrow (_msFactoryMgr.create("Property",
params));

Where _msFactoryMgr is a FactoryManager, a CORBA object, and PRoperty
is an structure defined in an IDL.
When I execute my code, I have got this error when I do the create:
     [java] java.lang.NullPointerException
     [java]     at
com.sun.corba.se.internal.corba.AnyImpl.write_value(AnyImpl.java:581)
     [java]     at
org.jacorb.orb.CDROutputStream.write_any(CDROutputStream.java:602)
     [java]     at
com.airbus.aspic.ParameterHelper.write(ParameterHelper.java:46)
     [java]     at
com.airbus.aspic.SequenceParametersHelper.write(SequenceParametersHelper.java:55)
     [java]     at
com.airbus.aspic._GenericFactoryStub.create(_GenericFactoryStub.java:78)
     [java]     at
com.airbus.framework.alf.FactoryManager.create(FactoryManager.java:59)

I'm blocked.. help!
Thanks!