ORA-00932: inconsistent datatypes

ORA-00932: inconsistent datatypes: expected - got CLOB
WHY THIS ERROR OCCURRING ?

WHEN WE EXECUTE THIS QUERY IT RESULTS TO AN ERROR (IE : ORA-00932)

SELECT TO_CLOB('DUMMY') FROM DUAL
UNION
SELECT TO_CLOB('DATA') FROM DUAL;

THIS ERROR CAN BE RESOLVED BY USING UNION ALL

EXAMPLE

SELECT TO_CLOB('DUMMY') FROM DUAL
UNION ALL
SELECT TO_CLOB('DATA') FROM DUAL;