diff --git a/ruff_lsp/server.py b/ruff_lsp/server.py
index f0e9c88..5e69051 100755
--- a/ruff_lsp/server.py
+++ b/ruff_lsp/server.py
@@ -428,7 +428,7 @@ def _create_single_cell_notebook_json(source: str) -> str:
             "cells": [
                 {
                     "cell_type": "code",
-                    "metadata": None,
+                    "metadata": {},
                     "outputs": [],
                     "source": source,
                 }