type conflict while calling a function module

I am creating a candidate registration form .

When i a loading the registration view i have called the auto generated id function which i have created.

But while i execute i get this error.

METHOD wddoinit .

DATA: count TYPE i VALUE 0,

wa_candidate TYPE zcanditate,

value TYPE char2 VALUE 'CN'.

CLEAR : wa_candidate.

" break-point.

wa_candidate-zdate = sy-datum.

CALL FUNCTION 'ZGET_NEXT_NUMBER'

EXPORTING

iv_object = value

IMPORTING

es_studentid = wa_candidate

EXCEPTIONS

nonumber = 1

notfound = 2

OTHERS = 3.

IF sy-subrc <> 0.

* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO

* WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.

ENDIF.

* SELECT COUNT(*) FROM zcanditate INTO count.

*

* count = count + 1.

* wa_candidate-candidate_id = count.

ENDMETHOD.

and the function that i have created is .

FUNCTION ZGET_NEXT_NUMBER.

*"----------------------------------------------------------------------

*"*"Local Interface:

*" IMPORTING

*" REFERENCE(IV_OBJECT) TYPE CHAR2

*" EXPORTING

*" REFERENCE(ES_STUDENTID) TYPE ZSTUDENT_ID

*" EXCEPTIONS

*" NONUMBER

*" NOTFOUND

*"----------------------------------------------------------------------

DATA: wa_zcadidate TYPE zcanditate.

IF sy-subrc EQ 0.

.

IF wa_zcadidate-candidate_id IS INITIAL.

wa_zcadidate-candidate_id = wa_zcadidate-from_number. ENDIF.

es_studentid = wa_zcadidate-candidate_id + 1. CONDENSE es_studentid.

wa_zcadidate-candidate_id = es_studentid.

IF wa_zcadidate-candidate_id LE wa_zcadidate-to_number.

CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT' EXPORTING

input = wa_zcadidate-candidate_id

IMPORTING

output = wa_zcadidate-candidate_id. CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT'

EXPORTING

input = es_studentid

IMPORTING

output = es_studentid.

UPDATE zcanditate FROM wa_zcadidate. ELSE.

RAISE nonumber.

ENDIF. ELSE.

RAISE notfound. ENDIF.

CALL FUNCTION 'DB_COMMIT'.

ENDFUNCTION.

I dont understand this error as i am new to this enviornment.