Results in ERROR: syntax error in tsquery:

SELECT *

FROM question

WHERE weighted_tsv @@ to_tsquery('Hur ofta');

But the query results in an error as follows:

ERROR:  function ts_query(unknown) does not exist

LINE 1: ...* from question where weighted_tsv @@ ts_query('...

HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

What is the reason I am getting this error I am not able to figure it out.

It would be of great help if anyone point me into right direction.

Thanks

Kiran