diff --git a/synapse/handlers/devicemessage.py b/synapse/handlers/devicemessage.py
index f7fad15c6..d996aa90b 100644
--- a/synapse/handlers/devicemessage.py
+++ b/synapse/handlers/devicemessage.py
@@ -17,7 +17,8 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.types import get_domain_from_id
+from synapse.api.errors import SynapseError
+from synapse.types import get_domain_from_id, UserID
 from synapse.util.stringutils import random_string
 
 
@@ -33,7 +34,7 @@ class DeviceMessageHandler(object):
         """
         self.store = hs.get_datastore()
         self.notifier = hs.get_notifier()
-        self.is_mine_id = hs.is_mine_id
+        self.is_mine = hs.is_mine
         self.federation = hs.get_federation_sender()
 
         hs.get_replication_layer().register_edu_handler(
@@ -52,6 +53,12 @@ class DeviceMessageHandler(object):
         message_type = content["type"]
         message_id = content["message_id"]
         for user_id, by_device in content["messages"].items():
+            # we use UserID.from_string to catch invalid user ids
+            if not self.is_mine(UserID.from_string(user_id)):
+                logger.warning("Request for keys for non-local user %s",
+                               user_id)
+                raise SynapseError(400, "Not a user here")
+
             messages_by_device = {
                 device_id: {
                     "content": message_content,
@@ -77,7 +84,8 @@ class DeviceMessageHandler(object):
         local_messages = {}
         remote_messages = {}
         for user_id, by_device in messages.items():
-            if self.is_mine_id(user_id):
+            # we use UserID.from_string to catch invalid user ids
+            if self.is_mine(UserID.from_string(user_id)):
                 messages_by_device = {
                     device_id: {
                         "content": message_content,
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index 668a90e49..5af8abf66 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -20,7 +20,7 @@ from canonicaljson import encode_canonical_json
 from twisted.internet import defer
 
 from synapse.api.errors import SynapseError, CodeMessageException
-from synapse.types import get_domain_from_id
+from synapse.types import get_domain_from_id, UserID
 from synapse.util.logcontext import preserve_fn, make_deferred_yieldable
 from synapse.util.retryutils import NotRetryingDestination
 
@@ -32,7 +32,7 @@ class E2eKeysHandler(object):
         self.store = hs.get_datastore()
         self.federation = hs.get_replication_layer()
         self.device_handler = hs.get_device_handler()
-        self.is_mine_id = hs.is_mine_id
+        self.is_mine = hs.is_mine
         self.clock = hs.get_clock()
 
         # doesn't really work as part of the generic query API, because the
@@ -70,7 +70,8 @@ class E2eKeysHandler(object):
         remote_queries = {}
 
         for user_id, device_ids in device_keys_query.items():
-            if self.is_mine_id(user_id):
+            # we use UserID.from_string to catch invalid user ids
+            if self.is_mine(UserID.from_string(user_id)):
                 local_query[user_id] = device_ids
             else:
                 remote_queries[user_id] = device_ids
@@ -170,7 +171,8 @@ class E2eKeysHandler(object):
 
         result_dict = {}
         for user_id, device_ids in query.items():
-            if not self.is_mine_id(user_id):
+            # we use UserID.from_string to catch invalid user ids
+            if not self.is_mine(UserID.from_string(user_id)):
                 logger.warning("Request for keys for non-local user %s",
                                user_id)
                 raise SynapseError(400, "Not a user here")
@@ -213,7 +215,8 @@ class E2eKeysHandler(object):
         remote_queries = {}
 
         for user_id, device_keys in query.get("one_time_keys", {}).items():
-            if self.is_mine_id(user_id):
+            # we use UserID.from_string to catch invalid user ids
+            if self.is_mine(UserID.from_string(user_id)):
                 for device_id, algorithm in device_keys.items():
                     local_query.append((user_id, device_id, algorithm))
             else: