1 files changed, 9 insertions, 7 deletions
diff --git a/synapse/config/cas.py b/synapse/config/cas.py
index b226890c2a..aaa7eba110 100644
--- a/synapse/config/cas.py
+++ b/synapse/config/cas.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from ._base import Config
+from ._base import Config, ConfigError
 
 
 class CasConfig(Config):
@@ -30,13 +30,15 @@ class CasConfig(Config):
 
         if self.cas_enabled:
             self.cas_server_url = cas_config["server_url"]
-            public_base_url = cas_config.get("service_url") or self.public_baseurl
-            if public_base_url[-1] != "/":
-                public_base_url += "/"
+
+            # The public baseurl is required because it is used by the redirect
+            # template.
+            public_baseurl = self.public_baseurl
+            if not public_baseurl:
+                raise ConfigError("cas_config requires a public_baseurl to be set")
+
             # TODO Update this to a _synapse URL.
-            self.cas_service_url = (
-                public_base_url + "_matrix/client/r0/login/cas/ticket"
-            )
+            self.cas_service_url = public_baseurl + "_matrix/client/r0/login/cas/ticket"
             self.cas_displayname_attribute = cas_config.get("displayname_attribute")
             self.cas_required_attributes = cas_config.get("required_attributes") or {}
         else: