ProgrammingError: column “xxxxx” of relation “xxxx” does not exist

I am using Openerp6.0 and I get the following error whenever i add a custom field in my openerp modules. Why does this error occurs..can anyone please help me??

for example:

ProgrammingError: column "x_brand" of relation "res_partner" does not exist
LINE 1: ...comment","website","lang","section_id","supplier","x_brand",...