Function module:G_Period_Get error exception1

Select - options:

s_zyear for s031-spmon.(MM.YYYY)

I need to convert the year and month to the fiscal year. And the coding is like this:

With debug, the exception 1 was issued, and I don't know why, Can anyone here help me??

please,thank you.

Data:
  LDF_DATUM TYPE SY_DATLO. 

LDF_DATUM+0(4) = S_ZYEAR-LOW+0(4).
LDF_DATUM+4(2) = S_ZYEAR-LOW+4(2).
LDF_DATUM+6(2) = '01'.

Call function 'G_Period_get'.
  Exporting
    company = p_bukrs
    date    = LDF_DATUM
    ledger  = 'LEDGER'
  Importing
    Period = GDF_Period
    Year   = Gdf_year
  Exception
    Lendger_not_assigned_to_company
= 1