SQL Error: ORA-00932: inconsistent datatypes: expected CHAR got NUMBER

I am trying to have a result 'none' every time it gives me a null result. Right now it is giving me a 0 for a null result. How could I have a row show me 'none' instead of a 0 for a null result.

I have tried TO_CHAR and TO_NUMBER for the sum and I can't get it to display 'none'…

CASE WHEN SUM(ENROLLED) = 0 THEN 'none' ELSE SUM(ENROLLED) END AS ENROLLED

so when try the above I get SQL Error: ORA-00932: inconsistent datatypes: expected CHAR got NUMBER

this is what I have

SELECT lt.STUDENT_ID,lt.FIRST_NAME, lt.LAST_NAME, CASE WHEN SUM(ENROLLED) = 0 THEN 'none' ELSE SUM(ENROLLED) END AS ENROLLED
FROM STUDENT lt
LEFT OUTER JOIN
(SELECT s.STUDENT_ID, e.ENROLL_DATE,COUNT(z.COURSE_NO)AS ENROLLED
FROM   STUDENT s
LEFT   JOIN ENROLLMENT e ON s.STUDENT_ID = e.STUDENT_ID
LEFT   JOIN SECTION z ON e.SECTION_ID = z.SECTION_ID
WHERE  s.PHONE LIKE '702%'
GROUP  BY s.STUDENT_ID, e.ENROLL_DATE) rt
ON lt.STUDENT_ID = rt.STUDENT_ID
WHERE lt.PHONE LIKE '702%'
GROUP BY lt.STUDENT_ID,lt.FIRST_NAME, lt.LAST_NAME,ENROLLMENTS;

instead of having

STUDENT_ID FIRST_NAME                LAST_NAME                 ENROLLED
---------- ------------------------- ------------------------- -----------
       253 Walter                    Boremmann                     1 
       396 James E.                  Norman                        0 
etc

I'd like to have it like this

STUDENT_ID FIRST_NAME                LAST_NAME                 ENROLLED
---------- ------------------------- ------------------------- -----------
       253 Walter                    Boremmann                     1 
       396 James E.                  Norman                        none