diff --git a/compiler/erg_compiler/lower.rs b/compiler/erg_compiler/lower.rs
index 8dd4170b..da4323d1 100644
--- a/compiler/erg_compiler/lower.rs
+++ b/compiler/erg_compiler/lower.rs
@@ -898,7 +898,10 @@ impl ASTLowerer {
                     Type::FreeVar(fv) if fv.is_unbound() => {
                         fv.get_sub().unwrap().return_t().unwrap().clone()
                     }
-                    other => todo!("{other:?}"),
+                    other => {
+                        log!(err "todo: {other}");
+                        return unreachable_error!(LowerErrors, LowerError, self.ctx);
+                    },
                 };
                 let arg_t = call.args.get(0).unwrap().ref_t();
                 self.ctx.sub_unify(arg_t, &ret_t, call.loc(), None)?;