diff --git a/sample/openssl_hostname_validation.c b/sample/openssl_hostname_validation.c
index b5adc67b..00e63d1e 100644
--- a/sample/openssl_hostname_validation.c
+++ b/sample/openssl_hostname_validation.c
@@ -41,6 +41,7 @@ SOFTWARE.
 
 #include <openssl/x509v3.h>
 #include <openssl/ssl.h>
+#include <string.h>
 
 #include "openssl_hostname_validation.h"
 #include "hostcheck.h"