diff --git a/src/ntlm.c b/src/ntlm.c
index 44421f5..28dff67 100644
--- a/src/ntlm.c
+++ b/src/ntlm.c
@@ -988,9 +988,9 @@ NTLM_INLINE(bool) generate_lm_hash(
 	keystr2_len = (password_len > 7) ? MIN(14, password_len) - 7 : 0;
 
 	for (i = 0; i < keystr1_len; i++)
-		keystr1[i] = (unsigned char)toupper(password[i]);
+		keystr1[i] = (unsigned char)toupper((unsigned char)password[i]);
 	for (i = 0; i < keystr2_len; i++)
-		keystr2[i] = (unsigned char)toupper(password[i+7]);
+		keystr2[i] = (unsigned char)toupper((unsigned char)password[i+7]);
 
 	/* DES encrypt the LM constant using the password as the key */
 	des_key_from_password(&key1, keystr1, keystr1_len);