SYS Error while trying to execute MERGE stmt thru dbms_parallel_execute pkg

I am trying to simulate a MERGE stmt execution thru DBMS_PARALLEL_EXECUTE package.

I have a million rows in a table(test_tab) and i copied and loaded another table (test_tab2)with 500 thousand rows and updated 1 of the columns with null values. When i run the MERGE stmt alone it works fine. When i run it thru the DBMS_PARALLEL_EXECUTE package it fails with the following error. The entire script is given below.

Thanks in advance for your help.

The scripts are attached in the file.