ProgrammingError: function sum(unknown) is not unique

Dear Community,

I got a very weird error, when trying to generate opening balances via centralised counterpart journal. It says :

"ProgrammingError: function sum(unknown) is not unique
LINE 1: SELECT SUM(E'debit') FROM account_move_line WHERE move_id=48...
               ^
HINT: Could not choose a best candidate function. You might need to add explicit type casts."

I am using openerp 5.0.11. I got the same error when trying to encode entries in centralised counterpart journal. when I did uncheck the centralised counterpart from journal, entries encoding went ok.

Help highly appreciated.

Thanks in advance