diff --git a/datasette/app.py b/datasette/app.py
index d94cd5a2..17fa06a5 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -191,7 +191,7 @@ class Datasette:
 
     def __init__(
         self,
-        files,
+        files=None,
         immutables=None,
         cache_headers=True,
         cors=False,
@@ -214,7 +214,7 @@ class Datasette:
         ), "config_dir= should be a pathlib.Path"
         self.pdb = pdb
         self._secret = secret or secrets.token_hex(32)
-        self.files = tuple(files) + tuple(immutables or [])
+        self.files = tuple(files or []) + tuple(immutables or [])
         if config_dir:
             self.files += tuple([str(p) for p in config_dir.glob("*.db")])
         if (
diff --git a/tests/test_internals_datasette.py b/tests/test_internals_datasette.py
index 56bc2fb4..692312a7 100644
--- a/tests/test_internals_datasette.py
+++ b/tests/test_internals_datasette.py
@@ -1,6 +1,7 @@
 """
 Tests for the datasette.app.Datasette class
 """
+from datasette.app import Datasette
 from itsdangerous import BadSignature
 from .fixtures import app_client
 import pytest
@@ -45,3 +46,19 @@ def test_sign_unsign(datasette, value, namespace):
 )
 def test_datasette_setting(datasette, setting, expected):
     assert datasette.setting(setting) == expected
+
+
+@pytest.mark.asyncio
+async def test_datasette_constructor():
+    ds = Datasette(memory=True)
+    databases = (await ds.client.get("/-/databases.json")).json()
+    assert databases == [
+        {
+            "name": "_memory",
+            "path": None,
+            "size": 0,
+            "is_mutable": False,
+            "is_memory": True,
+            "hash": None,
+        }
+    ]