diff --git a/hbmqtt/broker.py b/hbmqtt/broker.py
index 0d31f05..d2d4311 100644
--- a/hbmqtt/broker.py
+++ b/hbmqtt/broker.py
@@ -237,7 +237,13 @@ class Broker:
 
                     # SSL Context
                     sc = None
-                    if 'ssl' in listener and listener['ssl'].upper() == 'ON':
+
+                    # accept string "on" / "off" or boolean
+                    ssl_active = listener.get('ssl', False)
+                    if isinstance(ssl_active, str):
+                        ssl_active = ssl_active.upper() == 'ON'
+
+                    if ssl_active:
                         try:
                             sc = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
                             sc.load_cert_chain(listener['certfile'], listener['keyfile'])
@@ -745,4 +751,4 @@ class Broker:
                 return self._sessions[client_id][1]
             except KeyError:
                 pass
-        return None
\ No newline at end of file
+        return None