diff --git a/isisd/isis_circuit.c b/isisd/isis_circuit.c
index 4f7e2ce..fdff819 100644
--- a/isisd/isis_circuit.c
+++ b/isisd/isis_circuit.c
@@ -397,7 +397,7 @@
    */
   for (i = 0; i < strlen (ifp->name); i++)
     {
-      if (isdigit(ifp->name[i]))
+      if (isdigit((unsigned char)ifp->name[i]))
         {
           if (start < 0)
             {
diff --git a/zebra/rtadv.c b/zebra/rtadv.c
index 21ca6da..9a3fd26 100644
--- a/zebra/rtadv.c
+++ b/zebra/rtadv.c
@@ -1201,7 +1201,8 @@
 
   if (argc > 1)
     {
-      if ((isdigit(argv[1][0])) || strncmp (argv[1], "i", 1) == 0)
+      if ((isdigit((unsigned char)argv[1][0]))
+	  || strncmp (argv[1], "i", 1) == 0)
 	{
 	  if ( strncmp (argv[1], "i", 1) == 0)
 	    rp.AdvValidLifetime = UINT32_MAX;