Field symbol Runtime error

Hi,

I am using field symbol which has data already. I want to move that into a structure.. HOw do i acheive this ? I was trying the following but it is giving me a runtime error: "In unicode, the two structures have to be same structure"

FIELD-SYMBOLS: <ITAB> TYPE STANDARD TABLE,

DATA: LT_TAB TYPE TABLE OF mara,

LS_TAB LIKE LINE OF LT_TAB.

LOOP AT <ITAB> INTO LS_TAB.

APPEND LS_TAB TO LT_TAB.

ENDLOOP.

I had asked this ques earlier in the day as well and I recvd an answer "try assigning the value before the loop" but could not get it ?

any pointer to resolve this issue ??