diff --git a/src/script_opt/ZAM/Compile.h b/src/script_opt/ZAM/Compile.h
index c08873729f..ba9fd8f08f 100644
--- a/src/script_opt/ZAM/Compile.h
+++ b/src/script_opt/ZAM/Compile.h
@@ -362,7 +362,7 @@ private:
     int FrameSlot(const ID* id);
     int FrameSlotIfName(const Expr* e) {
         auto n = e->Tag() == EXPR_NAME ? e->AsNameExpr() : nullptr;
-        return n ? FrameSlot(n->Id()) : 0;
+        return n ? FrameSlot(n->Id()) : -1;
     }
 
     int FrameSlot(const NameExpr* id) { return FrameSlot(id->AsNameExpr()->Id()); }