Error Management with Lookup Temporary tables

Maybe there's a topic about this but i don't remember where is it...
example: i have a temporary table and i fill it with two or more loops on the real table. (here's a raw code sample)

Tbitem.setrange("no.",'aaa');
Tbitem.findset;
repeat
  tbitemtemp := Tbitem;
  if tbitemtemp.insert then;
until Tbitem.next = 0;

Tbitem.reset;
Tbitem.setrange("description",'*aaa*');
Tbitem.findset;
repeat
  tbitemtemp := Tbitem;
  if tbitemtemp.insert then;
until Tbitem.next = 0;

now, i'm gonna use this table in order to show a lookup in a request page.
moreover, if the user manually writes a value out of the range of this table, i have to give him an error.
My problem is that i still can't find a smart way to give a descriptive error to the user: here's my current version
-onlookup-

if page.runmodal(0,tbitemtemp) = action::lookupok then begin
  CDMyItemCodeVariable := tbitemptemp."no.";
  Myvalidatefunctionforitemcodevariable;
end;

-onvalidate-

//fill the buffer if the temporary table is still uninitialized
Myvalidatefunctionforitemcodevariable;

-Myvalidatefunctionforitemcodevariable-

tbitemtemp.get(CDMyItemCodeVariable);  //this instruction gives the error "item no. xxx does not exist" <--a bit confusing for the user, 
//as the item xxx DOES exist in the real table, but not in the temporary table
//other processing...

Thanks in advance (there's probably some error in the code, i wrote it in 5 minutes, sorry [-o< )