diff --git a/include/stdarg.h b/include/stdarg.h
index 9c41e6e..29df41d 100644
--- a/include/stdarg.h
+++ b/include/stdarg.h
@@ -49,6 +49,8 @@ static void *__va_arg_fp(__va_elem *ap, int sz, int align) {
             __va_arg_mem(ap, sizeof(ty), _Alignof(ty)));                \
   })
 
+#define va_copy(dest, src) ((dest)[0] = (src)[0])
+
 #define __GNUC_VA_LIST 1
 typedef va_list __gnuc_va_list;
 
diff --git a/test/test.h b/test/test.h
index 30bd7cd..af27f72 100644
--- a/test/test.h
+++ b/test/test.h
@@ -8,3 +8,4 @@ int strcmp(char *p, char *q);
 int strncmp(char *p, char *q, long n);
 int memcmp(char *p, char *q, long n);
 void exit(int n);
+int vsprintf();
diff --git a/test/varargs.c b/test/varargs.c
index 623a90f..2468fec 100644
--- a/test/varargs.c
+++ b/test/varargs.c
@@ -28,12 +28,14 @@ int sum2(int x, ...) {
   }
 }
 
-char *fmt(char *buf, char *fmt, ...) {
+void fmt(char *buf, char *fmt, ...) {
   va_list ap;
   va_start(ap, fmt);
-  vsprintf(buf, fmt, ap);
-  va_end(ap);
-  return buf;
+
+  va_list ap2;
+  va_copy(ap2, ap);
+  vsprintf(buf, fmt, ap2);
+  va_end(buf);
 }
 
 int main() {
@@ -42,6 +44,7 @@ int main() {
   ASSERT(21, sum2(1, 2.0, 3, 4.0, 5, 6.0, 0));
   ASSERT(21, sum2(1, 2.0, 3, 4.0, 5, 6.0, 0));
   ASSERT(210, sum2(1, 2.0, 3, 4.0, 5, 6.0, 7, 8.0, 9, 10.0, 11, 12.0, 13, 14.0, 15, 16.0, 17, 18.0, 19, 20.0, 0));
+  ASSERT(0, ({ char buf[100]; fmt(buf, "%d %d", 2, 3); strcmp(buf, "2 3"); }));
 
   printf("OK\n");
   return 0;