diff --git a/federationapi/federationapi_keys_test.go b/federationapi/federationapi_keys_test.go
index b95039636..4774c8820 100644
--- a/federationapi/federationapi_keys_test.go
+++ b/federationapi/federationapi_keys_test.go
@@ -68,6 +68,13 @@ func TestMain(m *testing.M) {
 			panic("can't create cache: " + err.Error())
 		}
 
+		// Create a temporary directory for JetStream.
+		d, err := ioutil.TempDir("./", "jetstream*")
+		if err != nil {
+			panic(err)
+		}
+		defer os.RemoveAll(d)
+
 		// Draw up just enough Dendrite config for the server key
 		// API to work.
 		cfg := &config.Dendrite{}
@@ -75,6 +82,8 @@ func TestMain(m *testing.M) {
 		cfg.Global.ServerName = gomatrixserverlib.ServerName(s.name)
 		cfg.Global.PrivateKey = testPriv
 		cfg.Global.JetStream.InMemory = true
+		cfg.Global.JetStream.TopicPrefix = string(s.name[:1])
+		cfg.Global.JetStream.StoragePath = config.Path(d)
 		cfg.Global.KeyID = serverKeyID
 		cfg.Global.KeyValidityPeriod = s.validity
 		cfg.FederationAPI.Database.ConnectionString = config.DataSource("file::memory:")
diff --git a/setup/jetstream/nats.go b/setup/jetstream/nats.go
index 5d8906601..6dbbd1f41 100644
--- a/setup/jetstream/nats.go
+++ b/setup/jetstream/nats.go
@@ -74,10 +74,7 @@ func setupNATS(cfg *config.JetStream, nc *natsclient.Conn) (nats.JetStreamContex
 			logrus.WithError(err).Fatal("Unable to get stream info")
 		}
 		if info == nil {
-			// Define a default subject if not already specified.
-			if len(stream.Subjects) == 0 {
-				stream.Subjects = []string{name}
-			}
+			stream.Subjects = []string{name}
 
 			// If we're trying to keep everything in memory (e.g. unit tests)
 			// then overwrite the storage policy.