Problem on INNER JOIN

DATA: BEGIN OF KOG,

ID TYPE ZSTUDENT_DATA-STUDENTID,

NAME TYPE ZSTUDENT_DATA-STUDENTNAME,

SUB_CODE TYPE ZSTUDENT_DATA-SUBJECTCODE,

MARKS TYPE ZSTUDENT_DATA-MARKS,

CITY TYPE ZSTUDENT_DATA-CITY,

SUB_NAME TYPE ZSUBJECT_DATA-SUBJECTNAME,

END OF KOG,

MYTABLE LIKE SORTED TABLE OF KOG WITH UNIQUE KEY ID.

SELECT ZSTU-STUDENTID ZSTU-STUDENTNAME ZSTU-SUBJECTCODE

ZSTU-MARKS ZSTU-CITY ZSUB-SUBJECTNAME

INTO CORRESPONDING FIELDS OF TABLE MYTABLE

FROM ( ZSTUDENT_DATA AS ZSTU INNER JOIN ZSUBJECT_DATA AS ZSUB

ON ZSUB-SUBJECTCODE EQ ZSTU-SUBJECTCODE )

WHERE ZSUB-SUBJECTNAME EQ 'MATH'.

LOOP AT MYTABLE INTO KOG.

AT FIRST.

WRITE :/1 'S.NO', 7 'STUDENT NAME', 24 'STUDENT ID', 36 'SUBJECT

CODE', 52 'MARKS', 61 'CITY', 74 'SUBJECT NAME'.

ENDAT.

ULINE.

WRITE : /1 SY-TABIX, 7 KOG-NAME, 25 KOG-ID, 39 KOG-SUB_CODE, 51

KOG-MARKS, 61 KOG-CITY, 74 KOG-SUB_NAME.

ENDLOOP.

HERE I AM GETTING SYNTAX ERROR --- Field "ZSUB-SUBJECTCODE" unknown