Error The integer table is readonly

The integer table is readonly, that is the error I get when i do a findset on a record of production bom lines.
I use the integer table as the table for my dataport (ie, the only dataitem is integer), the dataportfields are all global variables.

IF lv_prdbomh.GET(prodbom) THEN BEGIN
  lv_prdbomline.SETRANGE("Production BOM No.",prodbom);
  ii:=1;
  IF NOT lv_prdbomline.FINDFIRST THEN BEGIN     //<- error launches here
    REPEAT
    ...
   ii+=1

lv_prdbomh is a local variable record of production bom header
lv_prdbomline is a local vairable record of production bom line
ii is a local variable of integer

version is NAV 5.0 sp1 running on sqlserver