diff --git a/src/escpos/printer/lp.py b/src/escpos/printer/lp.py
index f928d38..e8b7bcf 100644
--- a/src/escpos/printer/lp.py
+++ b/src/escpos/printer/lp.py
@@ -188,6 +188,8 @@ class LP(Escpos):
         :param msg: arbitrary code to be printed
         :type msg: bytes
         """
+        assert self.device is not None
+        assert self.device.stdin is not None
         if self.device.stdin.writable():
             self.device.stdin.write(msg)
         else: