diff --git a/README.md b/README.md
index 3224c44..43b4a20 100644
--- a/README.md
+++ b/README.md
@@ -52,13 +52,20 @@ Add the MCP server to your Claude Code configuration by editing `~/.claude/setti
   "mcpServers": {
     "spotify": {
       "command": "/absolute/path/to/mcp-spotify/.venv/bin/python",
-      "args": ["/absolute/path/to/mcp-spotify/server.py"]
+      "args": ["/absolute/path/to/mcp-spotify/server.py"],
+      "env": {
+        "SPOTIPY_CLIENT_ID": "your_client_id_here",
+        "SPOTIPY_CLIENT_SECRET": "your_client_secret_here",
+        "SPOTIPY_REDIRECT_URI": "http://localhost:8888/callback"
+      }
     }
   }
 }
 ````
 
-**Note:** Replace `/absolute/path/to/mcp-spotify` with the actual path to your project. To find it, run `pwd` in the project directory:
+**Note:** Replace the path and fill in the env values with your actual Spotify app credentials. The `env` block is required because Claude Code launches the server as a subprocess from a different working directory where `load_dotenv()` may not find your `.env` file.
+
+Replace `/absolute/path/to/mcp-spotify` with the actual path to your project. To find it, run `pwd` in the project directory:
 
 ```bash
 cd /path/to/mcp-spotify
diff --git a/spotify_client.py b/spotify_client.py
index 704bb1e..27ef6dc 100644
--- a/spotify_client.py
+++ b/spotify_client.py
@@ -46,6 +46,8 @@ def get_client() -> spotipy.Spotify:
 
 
 def _format_duration(ms: int) -> str:
+    if ms is None:
+        return "unknown"
     seconds = ms // 1000
     minutes, seconds = divmod(seconds, 60)
     return f"{minutes}:{seconds:02d}"
@@ -154,6 +156,8 @@ def create_playlist(name: str, description: str = "", public: bool = False) -> d
 
 
 def add_tracks_to_playlist(playlist_id: str, track_uris: list[str]) -> dict:
+    if not track_uris:
+        raise ValueError("track_uris must not be empty")
     sp = get_client()
     for i in range(0, len(track_uris), 100):
         sp.playlist_add_items(playlist_id, track_uris[i : i + 100])