diff --git a/tests/storage/test__base.py b/tests/storage/test__base.py

index 8c3d2952bd..8fa305d18a 100644
--- a/tests/storage/test__base.py
+++ b/tests/storage/test__base.py
@@ -17,6 +17,8 @@
 from tests import unittest
 from twisted.internet import defer
 
+from synapse.util.async import ObservableDeferred
+
 from synapse.storage._base import Cache, cached
 
 
@@ -178,19 +180,20 @@ class CacheDecoratorTestCase(unittest.TestCase):
         self.assertTrue(callcount[0] >= 14,
             msg="Expected callcount >= 14, got %d" % (callcount[0]))
 
-    @defer.inlineCallbacks
     def test_prefill(self):
         callcount = [0]
 
+        d = defer.succeed(123)
+
         class A(object):
             @cached()
             def func(self, key):
                 callcount[0] += 1
-                return key
+                return d
 
         a = A()
 
-        a.func.prefill("foo", 123)
+        a.func.prefill("foo", ObservableDeferred(d))
 
-        self.assertEquals((yield a.func("foo")), 123)
+        self.assertEquals(a.func("foo").result, d.result)
         self.assertEquals(callcount[0], 0)