Can't add "functions" or "tools" parameter to request to chat gpt

API
1

First things first. I’m writing an app with Flutter and chat gpt. I’m using “model”: “gpt-3.5-turbo”. But every time I’m trying to include parameter “functions” or “tools” to my request I’m getting an error NoSuchMethodError: The method ‘’ was called on null. Without this parameter everything works as expected.
My request is

final response = await http.post(
    Uri.parse('https://api.openai.com/v1/chat/completions'),
    headers: {
      'Content-Type': 'application/json',
      'Authorization': 'Bearer .....'
    },
    body: jsonEncode(
      {
        "model": "gpt-3.5-turbo",
        "messages": dialogueHistory,
        "functions": [
          {
            "name": "throwDice",
            "description": "Throw dice to work out the result of a check.",
            "parameters": {
              "type": "object",
              "properties": {
                "modifier": {
                  "type": "number",
                  "description": "Check modifier that increases or decreases result of check."
                },
              },
            },
          }
        ],
        "function_call": "auto"
      },
    ),
  );

I have also tried

"tools": [
          {
            "type": "function",
            "function": {
              "name": "throwDice",
              "description": "Throw dice to work out the result of a check.",
              "parameters": {
                "type": "object",
                "properties": {
                  "modifier": {
                    "type": "number",
                    "description": "Check modifier that increases or decreases result of check."
                  },
                },

              }
            }
          }
        ]

But got the same error. What am I doing wrong?