diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 0346afb1b4..639795df28 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -412,6 +412,8 @@ class RoomMembershipRestServlet(ClientV1RestServlet):
             if "user_id" not in content:
                 raise SynapseError(400, "Missing user_id key.")
             state_key = content["user_id"]
+            # make sure it looks like a user ID; it'll throw if it's invalid.
+            UserID.from_string(state_key);
 
             if membership_action == "kick":
                 membership_action = "leave"