diff --git a/src/xymodem.h b/src/xymodem.h
index cd837e7..ef7f9a3 100644
--- a/src/xymodem.h
+++ b/src/xymodem.h
@@ -19,6 +19,8 @@
  * 02110-1301, USA.
  */
 
+#pragma once
+
 typedef enum {
     XMODEM_1K,
     XMODEM_CRC,