diff --git a/uplink/clients/io/interfaces.py b/uplink/clients/io/interfaces.py
index f2be6085..f4e4173a 100644
--- a/uplink/clients/io/interfaces.py
+++ b/uplink/clients/io/interfaces.py
@@ -245,6 +245,13 @@ def send(self, request):
         raise NotImplementedError
 
     def apply_callback(self, callback, response):
+        """
+        Invokes callback on the response.
+
+        Args:
+            callback (callable): a function that handles the response.
+            response: data returned from a server after request.
+        """
         raise NotImplementedError