diff --git a/cutils.c b/cutils.c
index 4d2e27c13a0924ae7df810797fca9acbbcd6a84f..a067cf45b3d0fb47c01318cff9723eb8789bedc6 100644
--- a/cutils.c
+++ b/cutils.c
@@ -294,7 +294,8 @@ int fcntl_setfl(int fd, int flag)
 int64_t strtosz_suffix(const char *nptr, char **end, const char default_suffix)
 {
     int64_t retval = -1;
-    char *endptr, c, d;
+    char *endptr;
+    unsigned char c, d;
     int mul_required = 0;
     double val, mul, integral, fraction;
 
@@ -314,7 +315,7 @@ int64_t strtosz_suffix(const char *nptr, char **end, const char default_suffix)
      */
     c = *endptr;
     d = c;
-    if (isspace(c) || c == '\0' || c == ',') {
+    if (qemu_isspace(c) || c == '\0' || c == ',') {
         c = 0;
         if (default_suffix) {
             d = default_suffix;
@@ -361,7 +362,7 @@ int64_t strtosz_suffix(const char *nptr, char **end, const char default_suffix)
      */
     if (c != 0) {
         endptr++;
-        if (!isspace(*endptr) && *endptr != ',' && *endptr != 0) {
+        if (!qemu_isspace(*endptr) && *endptr != ',' && *endptr != 0) {
             goto fail;
         }
     }