diff --git a/print-esp.c b/print-esp.c
index 6fabff1..cdeca9d 100644
--- a/print-esp.c
+++ b/print-esp.c
@@ -259,7 +259,7 @@ int esp_print_decrypt_buffer_by_ikev2(netdissect_options *ndo,
 		EVP_CIPHER_CTX_free(ctx);
 		(*ndo->ndo_error)(ndo, "can't allocate memory for decryption buffer");
 	}
-	EVP_Cipher(ctx, output_buffer, input_buffer, len);
+	EVP_Cipher(ctx, output_buffer, input_buffer, buffer_size);
 	EVP_CIPHER_CTX_free(ctx);
 
 	/*