diff --git a/nemu/tests/expr_test.c b/nemu/tests/expr_test.c
index 3cb30d5..0257e7c 100644
--- a/nemu/tests/expr_test.c
+++ b/nemu/tests/expr_test.c
@@ -29,11 +29,11 @@ void gen(char c) {
 }
 
 void gen_num(void) {
-  uint32_t num = rand();
+  int32_t num = rand();
   int len = 0, ref_len = 0;
   switch(rand() % 2) {
     case 0:
-      len = snprintf(buf_ptr, 100, "%u", num);
+      len = snprintf(buf_ptr, 100, "%d", num);
       ref_len = snprintf(ref_buf_ptr, 100, "%uU", num);
       break;
     case 1: