--- a/telnetlib.py
+++ b/telnetlib.py
@@ -421,6 +421,10 @@
         """Provide a callback function called after each receipt of a telnet option."""
         self.option_callback = callback
 
+    def filter_byte(self, c):
+        filter_list = (theNULL, b"\021")
+        return c in filter_list
+
     def process_rawq(self):
         """Transfer from raw queue to cooked queue.
 
@@ -433,9 +437,7 @@
             while self.rawq:
                 c = self.rawq_getchar()
                 if not self.iacseq:
-                    if c == theNULL:
-                        continue
-                    if c == b"\021":
+                    if self.filter_byte(c):
                         continue
                     if c != IAC:
                         buf[self.sb] = buf[self.sb] + c