diff --git a/library/pkwrite.c b/library/pkwrite.c
index f761ea0..29e172d 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -32,6 +32,8 @@
 #include "polarssl/asn1write.h"
 #include "polarssl/oid.h"
 
+#include <string.h>
+
 #if defined(POLARSSL_RSA_C)
 #include "polarssl/rsa.h"
 #endif