diff --git a/src/prompt_toolkit/contrib/telnet/protocol.py b/src/prompt_toolkit/contrib/telnet/protocol.py
index e56edd1ca..56e31035f 100644
--- a/src/prompt_toolkit/contrib/telnet/protocol.py
+++ b/src/prompt_toolkit/contrib/telnet/protocol.py
@@ -135,7 +135,7 @@ def ttype(self, data: bytes) -> None:
         """
         subcmd, data = data[0:1], data[1:]
         if subcmd == IS:
-            ttype = data.decode("ascii")
+            ttype = data.decode("ascii", errors="replace")
             self.ttype_received_callback(ttype)
         else:
             logger.warning("Received a non-IS terminal type Subnegotiation")