Derived property calling stored function throws StreamCorruptedException

I'm trying to improve performance by replacing a dynamic field (a transient getter with no underlying database representation) with a derived field so that I can use, e.g., Criteria to query my model. The original dynamic field was pretty simple:

Client resolveClient() {
    if (prevCall && prevCall.client) {
        return prevCall.client
    } else {
        return client
    }
}

I don't know how to reproduce that with a single MySQL statement, so I figured I would go ahead and stick it into a stored function, defined as follows:

CREATE FUNCTION `request_client`(requestId long) RETURNS varchar(255) CHARSET utf8
begin
    declare pci long;
    declare clientId long;
    declare clientName varchar(255);

    select request.prev_call_id
    from request
    where request.id = requestId
    into pci;

    if pci is not null then
      select call_history.client_id
      from call_history
      where call_history.call_id = pci
      into clientId;
    else
      select request.client_id
      from request
      where request.id = requestId
      into clientId;
    end if;

    select clients.client_name
    from clients
    where clients.client_id = clientId
    into clientName;

    return clientName;

  end;

And then I call that function in a derived field:

String derivedFieldName
static mapping = {
    derivedFieldName formula: '(select stored_function(id))'
}

The problem is that now when I run any query on the domain, even as simple as Request.list(), I get the following exception:

Class: java.io.StreamCorruptedException
Message: invalid stream header: 32303135

For extra fun, this is an abstract domain class. I don't know if that really makes any difference; it's still persisted to the database like any other domain, and I'm calling the query on the abstract class itself, not an implementation.

The most frustrating thing is that the derived field itself does work! I can successfully retrieve a client name using it; I just can't query the overall domain.

Finally, I am pretty confident that the derived property is the issue, as I have commented it out and can then successfully query the domain.