on line 605 of OPT.Mod typ := impCtxt.ref[-tag] read let's say, 19, i.

e. int64 typ, but because in procedure Close set to NIL everything upper
than FirstRef, which is 16, that's why obj.typ was not set in InStruct,
which caused crash later.
for now fixed by starting to NIL array from Comp + 1, not from FirstRef.
changing FirstRef causes crashes, which I did not investigate yet.

-- noch

src/voc/OPT.Mod

@@ -200,7 +200,7 @@ VAR i: INTEGER;
 BEGIN  (* garbage collection *)
 CloseScope;
 i := 0; WHILE i < maxImps DO GlbMod[i] := NIL; INC(i) END ;
-i := FirstRef; WHILE i < maxStruct DO impCtxt.ref[i] := NIL; impCtxt.old[i] := NIL; INC(i) END
+i := (*FirstRef*)Comp + 1; WHILE i < maxStruct DO impCtxt.ref[i] := NIL; impCtxt.old[i] := NIL; INC(i) END
 END Close;
 
 PROCEDURE FindImport*(mod: Object; VAR res: Object);