diff --git a/changelog.d/4740.bugfix b/changelog.d/4740.bugfix
new file mode 100644
index 0000000000..f82bb4227a
--- /dev/null
+++ b/changelog.d/4740.bugfix
@@ -0,0 +1 @@
+'event_id' is now a required parameter in federated state requests, as per the matrix spec.
diff --git a/synapse/federation/transport/server.py b/synapse/federation/transport/server.py
index 5ba94be2ec..ebb81be377 100644
--- a/synapse/federation/transport/server.py
+++ b/synapse/federation/transport/server.py
@@ -393,7 +393,7 @@ class FederationStateServlet(BaseFederationServlet):
         return self.handler.on_context_state_request(
             origin,
             context,
-            parse_string_from_args(query, "event_id", None),
+            parse_string_from_args(query, "event_id", None, required=True),
         )
 
 
@@ -404,7 +404,7 @@ class FederationStateIdsServlet(BaseFederationServlet):
         return self.handler.on_state_ids_request(
             origin,
             room_id,
-            parse_string_from_args(query, "event_id", None),
+            parse_string_from_args(query, "event_id", None, required=True),
         )