Trac Report Error

Error is:Report execution failed: ERROR: invalid input syntax for type timestamp: "" Report SQL is:SELECT id as ticket, summary, component, c.value as location, t.type AS type,   status, time AS createdFROM ticket t  LEFT OUTER JOIN ticket_custom c ON (t.id = c.ticket AND c.name = 'location')WHERE t.time BETWEEN EXTRACT(EPOCH FROM TIMESTAMP '$START 00:00:00') AND EXTRACT(EPOCH FROM TIMESTAMP '$END 23:59:59')ORDER BY time ASC, location, type