How to return null from a Dapper query rather than default(T)?

I'm using Dapper for some read-only database calls via a stored procedure. I've got a query that will either return 1 row or nothing.

I'm using Dapper like this:

using (var conn = new SqlConnection(ConnectionString))
{
    conn.Open();

    return conn.Query<CaseOfficer>("API.GetCaseOfficer", 
        new { Reference = reference }, 
        commandType: CommandType.StoredProcedure).FirstOrDefault();
}

The returned CaseOfficer object looks like this:

public class CaseOfficer
{
    public string Title { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public string Telephone { get; set; }
}

This is then returned through a ASP.NET Web API application as JSON.

When the stored procedure returns a result I get the following:

{
    title: "Mr",
    firstName: "Case",
    lastName: "Officer",
    email: "test@example.com",
    telephone: "01234 567890"
}

But when it returns nothing I get:

{
    title: null,
    firstName: null,
    lastName: null,
    email: null,
    telephone: null
}

How can I get Dapper to return null (so I can check and respond with 404), rather than the default(CaseOfficer)?