SQL & PL/SQL

Hello,

Here is a simplified explanation of my problem:

I have xml data stored in a table as a clob.

I am converting this to xmltype using this syntax:
var2 := xmltype(var1);

I am then trying to extract a specific element using this syntax:
var3 := var2.extract('/FIXML/CustDefAcctUpdAttributes/@Account').getstringval();

This then generates the following error:
ORA-30625: method dispatch on NULL SELF argument is disallowed

I have seen explanations of how to get such code to handle null elements.
However, in my case, the element in question is not null.
I wondered if anyone had any thoughts on why I might be getting this error?

Many Thanks,

Matthew