diff --git a/src/eddn/Gateway.py b/src/eddn/Gateway.py
index 3ae5463..2648546 100644
--- a/src/eddn/Gateway.py
+++ b/src/eddn/Gateway.py
@@ -11,7 +11,7 @@ import hashlib
 import logging
 import zlib
 from datetime import datetime
-from typing import Dict
+from typing import Callable, Dict
 from urllib.parse import parse_qs
 
 import gevent
@@ -384,7 +384,7 @@ class EnableCors(object):
     api = 2
 
     @staticmethod
-    def apply(fn, context):
+    def apply(self, fn: Callable, context: str):
         """
         Apply CORS headers to the calling bottle app.
 
diff --git a/src/eddn/Monitor.py b/src/eddn/Monitor.py
index feeb0c0..dc7bb5d 100644
--- a/src/eddn/Monitor.py
+++ b/src/eddn/Monitor.py
@@ -5,7 +5,7 @@ import collections
 import datetime
 import zlib
 from threading import Thread
-from typing import OrderedDict
+from typing import Callable, OrderedDict
 
 import gevent
 import mysql.connector as mariadb
@@ -285,7 +285,7 @@ class EnableCors(object):
     api = 2
 
     @staticmethod
-    def apply(fn, context):
+    def apply(self, fn: Callable, context: str):
         """
         Apply a CORS handler.
 
diff --git a/src/eddn/Relay.py b/src/eddn/Relay.py
index ecd29f5..3ff289d 100644
--- a/src/eddn/Relay.py
+++ b/src/eddn/Relay.py
@@ -8,6 +8,7 @@ import time
 import uuid
 import zlib
 from threading import Thread
+from typing import Callable
 
 # Logging has to be configured first before we do anything.
 logger = logging.getLogger(__name__)
@@ -194,7 +195,7 @@ class EnableCors(object):
     api = 2
 
     @staticmethod
-    def apply(fn, context):
+    def apply(self, fn: Callable, context: str):
         """
         Apply a CORS handler.