diff --git a/pc-bios/s390-ccw/libc.c b/pc-bios/s390-ccw/libc.c
index a786566c4c..3187923950 100644
--- a/pc-bios/s390-ccw/libc.c
+++ b/pc-bios/s390-ccw/libc.c
@@ -38,7 +38,7 @@ uint64_t atoui(const char *str)
     }
 
     while (*str) {
-        if (!isdigit(*str)) {
+        if (!isdigit(*(unsigned char *)str)) {
             break;
         }
         val = val * 10 + *str - '0';
diff --git a/pc-bios/s390-ccw/menu.c b/pc-bios/s390-ccw/menu.c
index 82a4ae6315..ce3815b201 100644
--- a/pc-bios/s390-ccw/menu.c
+++ b/pc-bios/s390-ccw/menu.c
@@ -134,7 +134,7 @@ static int get_index(void)
 
     /* Check for erroneous input */
     for (i = 0; i < len; i++) {
-        if (!isdigit(buf[i])) {
+        if (!isdigit((unsigned char)buf[i])) {
             return -1;
         }
     }