diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 2b51ad6d04..35544b19fd 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -45,8 +45,6 @@ CACHE_SIZE_FACTOR = float(os.environ.get("SYNAPSE_CACHE_FACTOR", 0.1))
 class Cache(object):
 
     def __init__(self, name, max_entries=1000, keylen=1, lru=True, tree=False):
-        max_entries = int(max_entries * CACHE_SIZE_FACTOR)
-
         if lru:
             cache_type = TreeCache if tree else dict
             self.cache = LruCache(
@@ -146,6 +144,8 @@ class CacheDescriptor(object):
     """
     def __init__(self, orig, max_entries=1000, num_args=1, lru=True, tree=False,
                  inlineCallbacks=False):
+        max_entries = int(max_entries * CACHE_SIZE_FACTOR)
+
         self.orig = orig
 
         if inlineCallbacks:
diff --git a/tests/storage/test_appservice.py b/tests/storage/test_appservice.py
index ed8af10d87..5734198121 100644
--- a/tests/storage/test_appservice.py
+++ b/tests/storage/test_appservice.py
@@ -35,7 +35,8 @@ class ApplicationServiceStoreTestCase(unittest.TestCase):
     def setUp(self):
         self.as_yaml_files = []
         config = Mock(
-            app_service_config_files=self.as_yaml_files
+            app_service_config_files=self.as_yaml_files,
+            event_cache_size=1,
         )
         hs = yield setup_test_homeserver(config=config)
 
@@ -109,7 +110,8 @@ class ApplicationServiceTransactionStoreTestCase(unittest.TestCase):
         self.as_yaml_files = []
 
         config = Mock(
-            app_service_config_files=self.as_yaml_files
+            app_service_config_files=self.as_yaml_files,
+            event_cache_size=1,
         )
         hs = yield setup_test_homeserver(config=config)
         self.db_pool = hs.get_db_pool()
@@ -438,7 +440,7 @@ class ApplicationServiceStoreConfigTestCase(unittest.TestCase):
         f1 = self._write_config(suffix="1")
         f2 = self._write_config(suffix="2")
 
-        config = Mock(app_service_config_files=[f1, f2])
+        config = Mock(app_service_config_files=[f1, f2], event_cache_size=1)
         hs = yield setup_test_homeserver(config=config, datastore=Mock())
 
         ApplicationServiceStore(hs)
@@ -448,7 +450,7 @@ class ApplicationServiceStoreConfigTestCase(unittest.TestCase):
         f1 = self._write_config(id="id", suffix="1")
         f2 = self._write_config(id="id", suffix="2")
 
-        config = Mock(app_service_config_files=[f1, f2])
+        config = Mock(app_service_config_files=[f1, f2], event_cache_size=1)
         hs = yield setup_test_homeserver(config=config, datastore=Mock())
 
         with self.assertRaises(ConfigError) as cm:
@@ -464,7 +466,7 @@ class ApplicationServiceStoreConfigTestCase(unittest.TestCase):
         f1 = self._write_config(as_token="as_token", suffix="1")
         f2 = self._write_config(as_token="as_token", suffix="2")
 
-        config = Mock(app_service_config_files=[f1, f2])
+        config = Mock(app_service_config_files=[f1, f2], event_cache_size=1)
         hs = yield setup_test_homeserver(config=config, datastore=Mock())
 
         with self.assertRaises(ConfigError) as cm: