diff --git a/tests/libtest/lib509.c b/tests/libtest/lib509.c
index 59ced355fa..755208b8d7 100644
--- a/tests/libtest/lib509.c
+++ b/tests/libtest/lib509.c
@@ -21,6 +21,8 @@
  ***************************************************************************/
 #include "test.h"
 
+#include <string.h>
+
 /*
  * This test uses these funny custom memory callbacks for the only purpose
  * of verifying that curl_global_init_mem() functionality is present in