How do I parse an object with a Dictionary from a yaml file?

Here's the general structure:

public struct Choice
{
    public string result;
    public string nextBranch;
}

public struct Branch
{
    public Dictionary<string, Choice> choices;
}

public class ChoiceTree
{
    public Dictionary<string, Branch> branches;
    // some other properties

    // some methods
}

And I'm currently manually filling up my ChoiceTree object with textboxes because this is a WPF project. I'd like, instead, to parse them from a YAML file. I've made this as a starting point. I have no idea if it's set up correctly, but I did at least use an online checker to make sure the syntax is correct:

branches:
  - Key: "main1"
    Value:
      choices:
        - Key: "few minutes"
          Value:
            result: "S"
            nextBranch: "refuse"
        - Key: "hold on"
          Value:
            result: "D"
            nextBranch: "refuse"
        - Key: "of course"
          Value:
            result: "LS"
            nextBranch: "accept"
        - Key: "fine"
          Value:
            result: "HD"
            nextBranch: "accept"
  - Key: "refuse"
    Value:
      choices:
        - Key: "reluctant"
          Value:
            result: "HH"
            nextBranch: "accept"
        - Key: "happily"
          Value:
            result: "L"
            nextBranch: "accept"
# more branches below...

I think the closest I've gotten so far is following this example. And by "closest" I mean, at least the code below compiles. But it does not work, and I have no idea why.

public class YamlChoiceTreeeParser
{
    public YamlChoiceTreeeParser(string filePath)
    {
        if (!File.Exists(filePath))
        {
            throw new Exception("YAML file not found");
        }
        using (StreamReader sr = new StreamReader(filePath))
        {
            var deserializer = new DeserializerBuilder()
                .Build();
            var choiceTree = deserializer.Deserialize<ChoiceTree>(sr);
        }
    }
}

I've tried a bunch of different variations on the yaml file, and a bunch of different examples, but I can't even step into the last line without an exception. (with the current code above, the exception I'm getting is "Expected 'MappingStart', got 'SequenceStart'"