Upgrade Form 6i to Form11g on calling RP2RRO.RP2RRO_RUN_PRODUCT

I was trying to upgrade a  application developed in Form6i to Form11g.  A menu.fmb with code

run_product(reports, 'HRM_manr',asynchronous,runtime,filesystem,TO_CHAR(NULL) in the menu item code (PL/SQL) suggested by the form migration assist to change to RP2RRO.RP2RRO_RUN_PRODUCT

I changed the code to

RP2RRO.RP2RRO_RUN_PRODUCT(REPORTS,'HRM_MANR',asynchronous,runtime,filesystem);

and compiled successfully; however, when I run through it on web, it returned me FRM-21011:PL/SQL unhandled exception ORA-06508

How do I further debug where went wrong ?

Regards

...Jane