diff --git a/library/cipher.c b/library/cipher.c
index 1827770..2463a61 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -320,7 +320,7 @@
             return( result );
 
         return mbedtls_chachapoly_update_aad( (mbedtls_chachapoly_context*) ctx->cipher_ctx,
-                                                          ad_len, ad );
+                                                          ad, ad_len );
     }
 #endif