diff --git a/src/socketio/async_namespace.py b/src/socketio/async_namespace.py
index 0a2e0515..89442aeb 100644
--- a/src/socketio/async_namespace.py
+++ b/src/socketio/async_namespace.py
@@ -29,7 +29,7 @@ async def trigger_event(self, event, *args):
 
         Note: this method is a coroutine.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             handler = getattr(self, handler_name)
             if asyncio.iscoroutinefunction(handler) is True:
@@ -194,7 +194,7 @@ async def trigger_event(self, event, *args):
 
         Note: this method is a coroutine.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             handler = getattr(self, handler_name)
             if asyncio.iscoroutinefunction(handler) is True:
diff --git a/src/socketio/namespace.py b/src/socketio/namespace.py
index ab4f69f8..3bf4f95b 100644
--- a/src/socketio/namespace.py
+++ b/src/socketio/namespace.py
@@ -21,7 +21,7 @@ def trigger_event(self, event, *args):
         method can be overridden if special dispatching rules are needed, or if
         having a single method that catches all events is desired.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             return getattr(self, handler_name)(*args)
 
@@ -152,7 +152,7 @@ def trigger_event(self, event, *args):
         method can be overridden if special dispatching rules are needed, or if
         having a single method that catches all events is desired.
         """
-        handler_name = 'on_' + event
+        handler_name = 'on_' + (event or '')
         if hasattr(self, handler_name):
             return getattr(self, handler_name)(*args)