Dapper multi-mapping error

I'm struggling to get multi mapping working in dapper.

Here's my code:

var contactDictionary = new Dictionary<Guid, Contact>();

IEnumerable<Contact> contacts = connection.Query<Contact, Activity, Contact>(
        @"SELECT ContactId, Title, FirstName, Surname, 
            AddressType.Type AS AddressType, Address.AddressLine1, Address.AddressLine2, Address.AddressLine3, Address.Town, Address.County, Address.Country, Address.Postcode,
            EmailAddressType.Type AS EmailAddressType, EmailAddress.Email AS EmailAddress,
            PhoneNumberType.Type As PhoneNumberType, PhoneNumber.Number AS PhoneNumber,
            Direction.Type AS Direction, Activity.Summary, Activity.DateCompleted
        FROM Contact
            LEFT JOIN Address ON Contact.ContactPK = Address.ContactPK
                LEFT JOIN AddressType ON Address.AddressTypePK = AddressType.AddressTypePK
            LEFT JOIN EmailAddress ON Contact.ContactPK = EmailAddress.ContactPK
                LEFT JOIN EmailAddressType ON EmailAddress.EmailAddressTypePK = EmailAddressType.EmailAddressTypePK
            LEFT JOIN PhoneNumber ON Contact.ContactPK = PhoneNumber.ContactPK
                LEFT JOIN PhoneNumberType ON PhoneNumber.PhoneNumberTypePK = PhoneNumberType.PhoneNumberTypePK
            LEFT JOIN Activity ON Contact.ContactPK = Activity.ContactPK
                LEFT JOIN Direction ON Activity.DirectionPK = Direction.DirectionPK
        WHERE Contact.ContactId = @ContactId
            AND Address.IsPrimary = 1 AND EmailAddress.IsPrimary = 1 AND PhoneNumber.IsPrimary = 1",
        param:new { ContactId = contactId },
        map:(c, a) =>
        {
            Contact contactEntry;

            if (!contactDictionary.TryGetValue(c.ContactId, out contactEntry))
            {
                contactEntry = c;
                contactEntry.Activities = new List<Activity>();
                contactDictionary.Add(contactEntry.ContactId, contactEntry);
            }

            contactEntry.Activities.Add(a);
            return contactEntry;
        },
        splitOn: "ContactId")
        .Distinct()
        .ToList();    

Here's the result of the SQL when run in SSMS:

Here's the error I'm getting when the code executes: When using the multi-mapping APIs ensure you set the splitOn param if you have keys other than Id Parameter name: splitOn

I'm really confused because I thought I had specified the splitOn parameter?

Any help would be appreciated.