diff --git a/lib/curl_ntlm_core.c b/lib/curl_ntlm_core.c
index b15215b2f9..25f2a3ad40 100644
--- a/lib/curl_ntlm_core.c
+++ b/lib/curl_ntlm_core.c
@@ -522,7 +522,7 @@ static void ascii_uppercase_to_unicode_le(unsigned char *dest,
 {
   size_t i;
   for(i = 0; i < srclen; i++) {
-    dest[2 * i] = (unsigned char)(toupper(src[i]));
+    dest[2 * i] = (unsigned char)(toupper((unsigned char)src[i]));
     dest[2 * i + 1] = '\0';
   }
 }