diff --git a/cbits/cryptonite_poly1305.h b/cbits/cryptonite_poly1305.h
index bfc6a53..f28d48d 100644
--- a/cbits/cryptonite_poly1305.h
+++ b/cbits/cryptonite_poly1305.h
@@ -32,6 +32,7 @@
 
 typedef struct
 {
+	uint8_t block[16]; /* previous blocks */
 } poly1305_ctx;
 
 typedef uint8_t poly1305_mac[16];