The unique option while creating a field is not working

Hi,
i created a field and i clicked the unique option for that field so that the user may not enter the same value once some particular value has be entered in that field. but if the user tries to enter the same value it leads to error.

"PDOException: SQLSTATE[22P02]: Invalid text representation: 7 ERROR: invalid input syntax for integer: "" LINE 4: WHERE (nid = '1') AND (cid = '1') AND (sid <> '') AND (data... ^: SELECT webform_submitted_data.sid AS sid FROM {webform_submitted_data} webform_submitted_data WHERE (nid = :db_condition_placeholder_0) AND (cid = :db_condition_placeholder_1) AND (sid <> :db_condition_placeholder_2) AND (data = :db_condition_placeholder_3) LIMIT 1 OFFSET 0; Array ( [:db_condition_placeholder_0] => 1 [:db_condition_placeholder_1] => 1 [:db_condition_placeholder_2] => [:db_condition_placeholder_3] => dfd ) in webform_validate_unique() (line 1067 of C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\test\sites\all\modules\webform\includes\webform.components.inc)."

i am getting this error if i use unique option. i am using postgres database. please give valuable suggestion to remove this error..
thanks in advance.