diff --git a/apps/nc/compat/readpassphrase.c b/apps/nc/compat/readpassphrase.c
index 0d04134..f3aa248 100644
--- a/apps/nc/compat/readpassphrase.c
+++ b/apps/nc/compat/readpassphrase.c
@@ -141,11 +141,11 @@ restart:
 			if (p < end) {
 				if ((flags & RPP_SEVENBIT))
 					ch &= 0x7f;
-				if (isalpha(ch)) {
+				if (isalpha((unsigned char)ch)) {
 					if ((flags & RPP_FORCELOWER))
-						ch = (char)tolower(ch);
+						ch = (char)tolower((unsigned char)ch);
 					if ((flags & RPP_FORCEUPPER))
-						ch = (char)toupper(ch);
+						ch = (char)toupper((unsigned char)ch);
 				}
 				*p++ = ch;
 			}