diff --git a/serialio/aio/posix.py b/serialio/aio/posix.py
index 3af9a81..e5caae1 100644
--- a/serialio/aio/posix.py
+++ b/serialio/aio/posix.py
@@ -269,7 +269,8 @@ async def close(self):
     #  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -  -
 
     @property
-    def in_waiting(self):
+    @ensure_open
+    async def in_waiting(self):
         """Return the number of bytes currently in the input buffer."""
         # ~ s = fcntl.ioctl(self.fd, termios.FIONREAD, TIOCM_zero_str)
         s = fcntl.ioctl(self.fd, TIOCINQ, TIOCM_zero_str)