diff --git a/photon/libc/stdio/vsnprintf.c b/photon/libc/stdio/vsnprintf.c
index bab175d..efb8af6 100644
--- a/photon/libc/stdio/vsnprintf.c
+++ b/photon/libc/stdio/vsnprintf.c
@@ -11,8 +11,10 @@ static void _out_buf(struct __printf_backend *bk, char ch)
 {
     struct __vsnprintf_arg *arg = bk->arg;
     if (arg->idx < arg->max) {
-        arg->buf[arg->idx++] = ch;
+        arg->buf[arg->idx] = ch;
     }
+
+    arg->idx++;
 }
 
 int vsnprintf(char *buf, size_t sz, const char *restrict format, va_list arg)
@@ -25,5 +27,10 @@ int vsnprintf(char *buf, size_t sz, const char *restrict format, va_list arg)
     bk.out = _out_buf;
     bk.arg = &bk_arg;
 
-    return __xprintf(&bk, format, arg);
+    __xprintf(&bk, format, arg);
+    if (!bk_arg.idx) {
+        return 0;
+    }
+
+    return bk_arg.idx - 1;
 }