diff --git a/synapse/types.py b/synapse/types.py
index 7b6ae44bd..f639651a7 100644
--- a/synapse/types.py
+++ b/synapse/types.py
@@ -22,7 +22,10 @@ Requester = namedtuple("Requester", ["user", "access_token_id", "is_guest"])
 
 
 def get_domain_from_id(string):
-    return string.split(":", 1)[1]
+    try:
+        return string.split(":", 1)[1]
+    except IndexError:
+        raise SynapseError(400, "Invalid ID: %r", string)
 
 
 class DomainSpecificString(