Why we can't provide a null parameter to method in CORBA?

If I have a method like this in my idl defined interface:
string echoString(in string mesg);

And if I call this method with a null parameter, ORB will fail in
serializing this parameter and report a org.omg.CORBA.BAD_PARAM.

But why CORBA has such a limitation? What's the history?
And how can I avoid this? Always check the parameter before CORBA
method invoke?

BTW:
My environment is JDK1.5 and the ORB is inline JAVA IDL.
The same thing happens in ORBACUS 4.3.