diff --git a/apps/prime.c b/apps/prime.c
index db17f59ed..5c731a7e0 100644
--- a/apps/prime.c
+++ b/apps/prime.c
@@ -47,9 +47,12 @@
  *
  */
 
+#include <string.h>
+
 #include "apps.h"
 #include <openssl/bn.h>
 
+
 #undef PROG
 #define PROG prime_main