TABLE_INVALID_INDEX run time error

hi i am writing this code for loading data of last two months from r/3.It is not giving any syntax Error. but when i schedule the load it throws a run time error . can any one suggest how to correct this error.

data: l_idx like sy-tabix.

DATA: lv_calmonth LIKE /BI0/SCALMONTH-CALMONTH.

DATA: lv_day TYPE DATS.

"previous month

lv_day = SY-DATUM.

lv_day+6(2) = '01'.

lv_day = lv_day - 1.

lv_calmonth = lv_day(6).

READ TABLE l_t_range with key fieldname = 'CALMONTH'.

l_idx = sy-tabix.

MOVE lv_calmonth TO l_t_range-low.

MODIFY l_t_range INDEX l_idx.

"previous month - 1

lv_day+6(2) = '01'.

lv_day = lv_day - 1.

lv_calmonth = lv_day(6).

MOVE lv_calmonth TO l_t_range-low.

APPEND l_t_range.

p_subrc = 0.