diff --git a/os/access.c b/os/access.c
index 8305d571c..7fd85a5a7 100644
--- a/os/access.c
+++ b/os/access.c
@@ -978,7 +978,7 @@ ResetHosts(const char *display)
                 *ptr = 0;
             hostlen = strlen(ohostname) + 1;
             for (i = 0; i < hostlen; i++)
-                lhostname[i] = tolower(ohostname[i]);
+                lhostname[i] = tolower((unsigned char)ohostname[i]);
             hostname = ohostname;
             if (!strncmp("local:", lhostname, 6)) {
                 family = FamilyLocalHost;