In the QueryMultiple method when not reading all of the refcursors of a PostgreSQL function the Dispose method of GridReader is invoked. Since the reader is not closed, command.Cancel() is called. (https://github.com/StackExchange/dapper-dot-net/blob/master/Dapper%20NET40/SqlMapper.cs#L4497)

Reading the user manual of Npgsql (http://www.npgsql.org/doc/manual-2.2.html) regarding the Cancel method it states:
"Npgsql is able to ask the server to cancel commands in progress. To do this, call the NpgsqlCommand’s Cancel method. Note that another thread must handle the request as the main thread will be blocked waiting for command to finish. Also, the main thread will raise an exception as a result of user cancellation. (The error code is 57014.)"

This causes code like this to throw exception if the first refcursor is empty:

using (var multipleResults = connection.QueryMultiple("schema.getuserbysocialsecuricy", new { socialSecurityNumber }))
{
    var client = multipleResults.Read<Client>().SingleOrDefault();

    if (client != null)
    {
        client.Address = multipleResults.Read<Address>().Single();
    }

    return client;
}

Can you confirm if this is the desired result or is it an issue? Thank you!