diff --git a/nemu/tests/expr_test.c b/nemu/tests/expr_test.c
index a31dd8f..0da2996 100644
--- a/nemu/tests/expr_test.c
+++ b/nemu/tests/expr_test.c
@@ -177,8 +177,8 @@ START_TEST(test_expr_plain_register) {
     gpr(i) = i;
     yy_scan_string(buf);
     result = yyparse(&value);
-    ck_assert_msg(result == 0, "expr = %s\n", buf);
     yylex_destroy();
+    ck_assert_msg(result == 0, "expr = %s\n", buf);
 
     ck_assert(value == i);
     for (j = 1; j < 10; j++) {