diff --git a/synapse/handlers/cas_handler.py b/synapse/handlers/cas_handler.py
index 85304c4f43..f4ea0a9767 100644
--- a/synapse/handlers/cas_handler.py
+++ b/synapse/handlers/cas_handler.py
@@ -203,32 +203,57 @@ class CasHandler:
             args["session"] = session
         username, user_display_name = await self._validate_ticket(ticket, args)
 
-        localpart = map_username_to_mxid_localpart(username)
-        user_id = UserID(localpart, self._hostname).to_string()
-        registered_user_id = await self._auth_handler.check_user_exists(user_id)
+        # Pull out the user-agent and IP from the request.
+        user_agent = request.get_user_agent("")
+        ip_address = self.hs.get_ip_from_request(request)
+
+        # Get the matrix ID from the CAS username.
+        user_id = await self._map_cas_user_to_matrix_user(
+            username, user_display_name, user_agent, ip_address
+        )
 
         if session:
-            # If there's a session then the user must already exist.
-            assert registered_user_id
-
             await self._auth_handler.complete_sso_ui_auth(
-                registered_user_id, session, request,
+                user_id, session, request,
             )
         else:
             # If this not a UI auth request than there must be a redirect URL.
             assert client_redirect_url
 
-            if not registered_user_id:
-                # Pull out the user-agent and IP from the request.
-                user_agent = request.get_user_agent("")
-                ip_address = self.hs.get_ip_from_request(request)
-
-                registered_user_id = await self._registration_handler.register_user(
-                    localpart=localpart,
-                    default_display_name=user_display_name,
-                    user_agent_ips=[(user_agent, ip_address)],
-                )
-
             await self._auth_handler.complete_sso_login(
-                registered_user_id, request, client_redirect_url
+                user_id, request, client_redirect_url
             )
+
+    async def _map_cas_user_to_matrix_user(
+        self,
+        remote_user_id: str,
+        display_name: Optional[str],
+        user_agent: str,
+        ip_address: str,
+    ) -> str:
+        """
+        Given a CAS username, retrieve the user ID for it and possibly register the user.
+
+        Args:
+            remote_user_id: The username from the CAS response.
+            display_name: The display name from the CAS response.
+            user_agent: The user agent of the client making the request.
+            ip_address: The IP address of the client making the request.
+
+        Returns:
+             The user ID associated with this response.
+        """
+
+        localpart = map_username_to_mxid_localpart(remote_user_id)
+        user_id = UserID(localpart, self._hostname).to_string()
+        registered_user_id = await self._auth_handler.check_user_exists(user_id)
+
+        # If the user does not exist, register it.
+        if not registered_user_id:
+            registered_user_id = await self._registration_handler.register_user(
+                localpart=localpart,
+                default_display_name=display_name,
+                user_agent_ips=[(user_agent, ip_address)],
+            )
+
+        return registered_user_id