ORA-39001: invalid argument value when using a variable to assign the value for dbms_datapump.metada

Hi

I've been trying to get datapump working to an acceptable speed for use by our testing team.  After spending a couple of weeks using the expdp and impdp command line tools, I found that using the DBMS_DATAPUMP API gave far better performance.  I am now trying to incorporate this into an existing package in order to roll out the functionality to our users.  However, when trying to run the sub program in the package I get the following error:

ORA-39001: invalid argument value

I found that this relates to the following part of the code:

                               dbms_datapump.metadata_filter(