Unable to display HTML/XML data on email notification body

 Hi,
I a trying to display the clob column (which contains HTML/xml tags and data) in a workflow notification email body.

I created the procedures and functions that will return me the HTML/xml data value as a single column which is stored in the CLOB column and then used "Wf_Engine.setitemattrtext" to set attribute value("ZZ_COST_MESG_DETAIL").
In Workflow when I created a attribute "ZZ_COST_MESG_DETAIL" of type varchar2 and size 4000, I get the following error:

When I run this workflow I am getting the following error:
Error Name = -6502
Error Message = ORA-06502: PL/SQL: numeric or value error