Serialize decimal to string in Newtonsoft.Json

StringDecimalConverter.cs

public class StringDecimalConverter : JsonConverter
{
    public override bool CanRead
    {
        get
        {
            return false;
        }
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(decimal) || objectType == typeof(decimal?);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(((decimal)value).ToString(CultureInfo.InvariantCulture));
    }
}

Should you not check for null in your WriteJson method? As you also cover nullable decimals.