diff --git a/hw/xfree86/common/xf86Bus.c b/hw/xfree86/common/xf86Bus.c
index 5e5cdb1a6..7d778c4dd 100644
--- a/hw/xfree86/common/xf86Bus.c
+++ b/hw/xfree86/common/xf86Bus.c
@@ -266,7 +266,7 @@ StringToBusType(const char *busID, const char **retID)
     BusType ret = BUS_NONE;
 
     /* If no type field, Default to PCI */
-    if (isdigit(busID[0])) {
+    if (isdigit((unsigned char)busID[0])) {
         if (retID)
             *retID = busID;
         return BUS_PCI;