Dapper with postgresql error (sequence contains more than one element)

I am getting this error when I run an integration test against my Postgresql DB using a stored procedure.

Result Message: System.InvalidOperationException : Sequence contains more than one element

Here is the repo file:

  public card_view SelectView(int card_id)
        {
            using (var connection = new NpgsqlConnection(ConfigurationSettings.GetConnectionString()))
            {
                var p = new DynamicParameters();
                p.Add("@card_id", card_id);

                using (var multi = connection.QueryMultiple("f_card_select_view", p, commandType: CommandType.StoredProcedure))
                {
                    card_view view = multi.Read<card_view>().Single();
                    view.Categories = multi.Read<category>().ToList();
                    view.Modifiers = multi.Read<card_modifier_view>().ToList();

                    return view;
                }
            }
        }

The card test file:

   [Test]
    public void SelectViewTest()
    {
        var repo = new CardRepository();
        var result = repo.SelectView(31);  // witch

        Assert.AreEqual(2, result.Categories.Count);
        Assert.AreEqual(2, result.Modifiers.Count);
    }

Cardview file:

public class card_view
    {
        public int card_id { get; set; }
        public int cardset_id { get; set; }
        public string cardset_title { get; set; }
        public string image_path { get; set; }
        public string cardset_name { get; set; }
        public int card_cost { get; set; }

        public List<card_modifier_view> Modifiers { get; set; }
        public List<category> Categories { get; set; }
    }

cardmodifierview file:

public class card_modifier_view
    {
        public int card_modifier_id { get; set; }
        public int card_id { get; set; }
        public int modifier_type_id { get; set; }
        public int? modifier_value { get; set; }
        public string instruction_text { get; set; }
        public string modifier_type_name { get; set; }
    }

The DB function (The original SQL was T-SQL and I have done my best to translate it into regular SQL for postgres.)

CREATE FUNCTION f_card_select_view (card_id int)
RETURNS TABLE(card_id bigint, modifier_type_id integer, 
instruction_text integer, modifier_type_name integer, card_modifier character varying, modifier_type character varying)
AS $$
SELECT card_id,cardset.cardset_id,card_title,image_path,card_cost,cardset_name
FROM card 
INNER JOIN cardset ON card.cardset_id = cardset.cardset_id
WHERE card_id = @card_id;
SELECT  card.category_id,category_name
FROM category card
INNER JOIN card_category ON card.category_id = card_category.category_id
WHERE card_category.card_id = @card_id;
SELECT f_card_modifier_selectby_card_id (@card_id);
$$ LANGUAGE sql;

Here is f_card_modifier_selectby_card_id:

CREATE FUNCTION f_card_modifier_selectby_card_id(card_id int)
RETURNS TABLE(
card_id bigint,
modifier_type_id int,
instruction_text int,
modifier_type_name int,
card_modifier varchar,
modifier_type varchar
)
AS $$

SELECT
card_modifier_id,
card_id,
card_modifier.modifier_type_id,
modifier_value,
instruction_text,
modifier_type_name
FROM card_modifier INNER JOIN modifier_type ON card_modifier.modifier_type_id = modifier_type.modifier_type_id
WHERE card_id = card_id

$$ LANGUAGE sql;