diff --git a/libcpuid/libcpuid_util.c b/libcpuid/libcpuid_util.c
index 72077db..eca536a 100644
--- a/libcpuid/libcpuid_util.c
+++ b/libcpuid/libcpuid_util.c
@@ -28,6 +28,7 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include <string.h>
+#include <ctype.h>
 #include "libcpuid.h"
 #include "libcpuid_util.h"