ACCESS 97

I am trying to update 2 fields in a table. Both of the fields are numerics. The first of these fields updates but the second field (CYCLE) gives me a field not updatable error. The form is based on a query linking 2 tables on the trknum field.

DoCmd.RunSQL ("update prototype set trknum = " & mtxt0 & ", cycle = " & mtxt1 & ";")

I'm not sure what settings to check and or change to correct this and make the field updatable. Any ideas?