DBMS_DATAPUMP with DBMS_SCHEDULER

Oracle version 11.2.0.2 64 bit.

Operating systen Linux x86_64.

I created a PL/SQL block to run DBMS_DATAPUMP, importing over a database link. Success.

I put the block into a procedure within a package and ran that. Success.

I created a job with DBMS_SCHEDULER to call the procedure and ran that using DBMS_SCHEDULER.RUN_JOB. Success.

When the job runs at scheduled time I get:-

ORA-39001: invalid argument value
ORA-06512: at "SYS.DBMS_SYS_ERROR", line 79
ORA-06512: at "SYS.DBMS_DATAPUMP", line 3507
ORA-06512: at "SYS.DBMS_DATAPUMP", line 5296
ORA-06512: at "NPM.IMP", line 42
ORA-06512: at "NPM.IMP", line 134

Any Ideas?