diff --git a/lib/idn_win32.c b/lib/idn_win32.c
index 6ea3dc0..68accdc 100644
--- a/lib/idn_win32.c
+++ b/lib/idn_win32.c
@@ -61,6 +61,7 @@
 
 int curl_win32_ascii_to_idn(const char *in, size_t in_len, char **out_utf8)
 {
+  (void)in_len; /* unused */
   if(in) {
     WCHAR unicode[IDN_MAX_LENGTH];