diff --git a/ptstream.h b/ptstream.h
index ee36af2..e944c54 100644
--- a/ptstream.h
+++ b/ptstream.h
@@ -19,6 +19,7 @@
 
 /* ptstream.h */
 
+#include <stddef.h>
 #ifdef USE_SSL
 #include <openssl/crypto.h>
 #include <openssl/x509.h>