DSUM gives a null value

[Ankit Mehta]

I am trying to use a DSUM command to add production for 4 weeks to display it in the production for month. First I wrote a subquery in the set command. I got the error operation must run an updateable query. Then I tried the same with a DSUM. I get a null value. I am new to access please help me out. Any help is appreciated.

UPDATE weekly_schuler_prod_nrft_tbl SET schuler_month_prod = (DSUM("schuler_completions","weekly_schuler_prod_nrft_tbl","week" In ("40","41","42","43")))
WHERE [week]="43";