diff --git a/synapse/appservice/api.py b/synapse/appservice/api.py
index 775417eb21..cc4af23962 100644
--- a/synapse/appservice/api.py
+++ b/synapse/appservice/api.py
@@ -67,6 +67,8 @@ class ApplicationServiceApi(SimpleHttpClient):
 
     @defer.inlineCallbacks
     def query_user(self, service, user_id):
+        if service.url is None:
+            defer.returnValue(False)
         uri = service.url + ("/users/%s" % urllib.quote(user_id))
         response = None
         try:
@@ -86,6 +88,8 @@ class ApplicationServiceApi(SimpleHttpClient):
 
     @defer.inlineCallbacks
     def query_alias(self, service, alias):
+        if service.url is None:
+            defer.returnValue(False)
         uri = service.url + ("/rooms/%s" % urllib.quote(alias))
         response = None
         try:
@@ -113,6 +117,8 @@ class ApplicationServiceApi(SimpleHttpClient):
             raise ValueError(
                 "Unrecognised 'kind' argument %r to query_3pe()", kind
             )
+        if service.url is None:
+            defer.returnValue([])
 
         uri = "%s%s/thirdparty/%s/%s" % (
             service.url,
@@ -145,6 +151,9 @@ class ApplicationServiceApi(SimpleHttpClient):
             defer.returnValue([])
 
     def get_3pe_protocol(self, service, protocol):
+        if service.url is None:
+            defer.returnValue({})
+
         @defer.inlineCallbacks
         def _get():
             uri = "%s%s/thirdparty/protocol/%s" % (
@@ -166,6 +175,9 @@ class ApplicationServiceApi(SimpleHttpClient):
 
     @defer.inlineCallbacks
     def push_bulk(self, service, events, txn_id=None):
+        if service.url is None:
+            defer.returnValue(True)
+
         events = self._serialize(events)
 
         if txn_id is None:
diff --git a/synapse/config/appservice.py b/synapse/config/appservice.py
index dfe43b0b4c..d7537e8d44 100644
--- a/synapse/config/appservice.py
+++ b/synapse/config/appservice.py
@@ -86,7 +86,7 @@ def load_appservices(hostname, config_files):
 
 def _load_appservice(hostname, as_info, config_filename):
     required_string_fields = [
-        "id", "url", "as_token", "hs_token", "sender_localpart"
+        "id", "as_token", "hs_token", "sender_localpart"
     ]
     for field in required_string_fields:
         if not isinstance(as_info.get(field), basestring):
@@ -94,6 +94,14 @@ def _load_appservice(hostname, as_info, config_filename):
                 field, config_filename,
             ))
 
+    # 'url' must either be a string or explicitly null, not missing
+    # to avoid accidentally turning off push for ASes.
+    if (not isinstance(as_info.get("url"), basestring) and
+            as_info.get("url", "") is not None):
+        raise KeyError(
+            "Required string field or explicit null: 'url' (%s)" % (config_filename,)
+        )
+
     localpart = as_info["sender_localpart"]
     if urllib.quote(localpart) != localpart:
         raise ValueError(
@@ -132,6 +140,13 @@ def _load_appservice(hostname, as_info, config_filename):
         for p in protocols:
             if not isinstance(p, str):
                 raise KeyError("Bad value for 'protocols' item")
+
+    if as_info["url"] is None:
+        logger.info(
+            "(%s) Explicitly empty 'url' provided. This application service"
+            " will not receive events or queries.",
+            config_filename,
+        )
     return ApplicationService(
         token=as_info["as_token"],
         url=as_info["url"],