Help needed regarding SUM keyword in an ITAB loop

Hello,

I am maintaining a code as given below.

LOOP AT itab INTO totwa.

SUM.

ENDLOOP.

totwa is defined as a workarea which is needed to hold the sum totals of the fields in the itab. According to the documentation of SUM, it should calculate the sum totals and should put the totals in the workarea mentioned. But somehow its not calculating the value. Could anyone please help me in resolving this issue. Please guide me with your thoughts as to what might have gone wrong.

Note: the itab fields are of type P.

Thanks in advance

Sudha Naik