diff --git a/datasette/app.py b/datasette/app.py
index c052be58..6b889f08 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -218,6 +218,8 @@ class Datasette:
         self.config_dir = config_dir
         self.pdb = pdb
         self._secret = secret or secrets.token_hex(32)
+        if files is not None and isinstance(files, str):
+            raise ValueError("files= must be a list of paths, not a string")
         self.files = tuple(files or []) + tuple(immutables or [])
         if config_dir:
             db_files = []
diff --git a/tests/test_internals_datasette.py b/tests/test_internals_datasette.py
index 1b4732af..e2cb0870 100644
--- a/tests/test_internals_datasette.py
+++ b/tests/test_internals_datasette.py
@@ -130,7 +130,14 @@ async def test_datasette_ensure_permissions_check_visibility(
 @pytest.mark.asyncio
 async def test_datasette_render_template_no_request():
     # https://github.com/simonw/datasette/issues/1849
-    ds = Datasette([], memory=True)
+    ds = Datasette(memory=True)
     await ds.invoke_startup()
     rendered = await ds.render_template("error.html")
     assert "Error " in rendered
+
+
+def test_datasette_error_if_string_not_list(tmpdir):
+    # https://github.com/simonw/datasette/issues/1985
+    db_path = str(tmpdir / "data.db")
+    with pytest.raises(ValueError):
+        ds = Datasette(db_path)