Error in ORDER BY CASE WHEN Clause

SELECT "RV_PROTOCOL_BASIC"."PROTOCOL_NO" AS protocol_no,
             "RV_PROTOCOL_BASIC"."PI_NAMES" AS principal_investigator,
             "RV_PROTOCOL_BASIC"."STATUS" AS current_status,
            "RV_PROTOCOL_BASIC"."SPONSOR" AS sponsor,
             CAST("RV_PROTOCOL_BASIC"."IRB_EXPIRATION" AS DATE) AS expiration_date,
             "RV_PROTOCOL_BASIC"."IRB_EXPIRATION_NA" AS expiration_na
 FROM "RV_PROTOCOL_BASIC" JOIN "RV_PROTOCOL_DETAILS"
     ON "RV_PROTOCOL_BASIC"."PROTOCOL_NO" = "RV_PROTOCOL_DETAILS"."PROTOCOL_NO"
 WHERE ("RV_PROTOCOL_BASIC"."STATUS" = 'OPEN TO ACCRUAL'
     OR "RV_PROTOCOL_BASIC"."STATUS" = 'CLOSED TO ACCRUAL'
     OR "RV_PROTOCOL_BASIC"."STATUS" = 'SUSPENDED')
     AND
         ("RV_PROTOCOL_BASIC"."IRB_EXPIRATION" IS NULL
         OR  "RV_PROTOCOL_BASIC"."IRB_EXPIRATION" < $P{currentDate}
         OR  "RV_PROTOCOL_BASIC"."IRB_EXPIRATION_NA" = 'Y')
     AND $X{IN, "RV_PROTOCOL_BASIC"."PI_NAMES", selectedPI}
     AND $X{IN, "RV_PROTOCOL_BASIC"."SPONSOR", selectedSponsor}
     AND $X{IN, "RV_PROTOCOL_DETAILS"."DT4_REPORT_TYPE", dt4Type}
 ORDER BY
     CASE WHEN $P!{sortBy} = 'Protocol no.' AND $P!{sortOrder} = 'Ascending' THEN protocol_no ASC
         WHEN $P!{sortBy} = 'Pis' AND $P!{sortOrder} = 'Ascending' THEN principal_investigator ASC
         WHEN $P!{sortBy} = 'Current status' AND $P!{sortOrder} = 'Ascending' THEN current_status ASC
         WHEN $P!{sortBy} = 'Sponsor' AND $P!{sortOrder} = 'Ascending' THEN sponsor ASC
         WHEN $P!{sortBy} = 'Expiration date' AND $P!{sortOrder} = 'Ascending' THEN expiration_date ASC
         WHEN $P!{sortBy} = 'Expiration n/a?' AND $P!{sortOrder} = 'Ascending' THEN expiration_na ASC
         WHEN $P!{sortBy} = 'Protocol no.' AND $P!{sortOrder} = 'Descending' THEN protocol_no DESC
         WHEN $P!{sortBy} = 'Pis' AND $P!{sortOrder} = 'Descending' THEN principal_investigator DESC
         WHEN $P!{sortBy} = 'Current status' AND $P!{sortOrder} = 'Descending' THEN current_status DESC
         WHEN $P!{sortBy} = 'Sponsor' AND $P!{sortOrder} = 'Descending' THEN sponsor DESC
         WHEN $P!{sortBy} = 'Expiration date' AND $P!{sortOrder} = 'Descending' THEN expiration_date DESC
         WHEN $P!{sortBy} = 'Expiration n/a?' AND $P!{sortOrder} = 'Descending' THEN expiration_na DESC
     END