Workflow presentation action causes "processing presentation, please wait"

I have a workflow with a string field that I want to offer as a dropdown of results from a SQL database. I've created an action to query the database that is passed a SQL:Database value. The action script is below:

// DO query on MGMTDB

var results = new Array();

var SingleResult;

var query = "SELECT TemplateOS,TemplateVersion FROM TableName";

var QueryResults = MGMTDB.readCustomQuery(query);

for each (SingleResult in QueryResults)

{

    var template = SingleResult.getProperty("TemplateOS") + " " + SingleResult.getProperty("TemplateVersion");

    //System.debug("getTemplateNames:: " + template);

    results.push(template);

}

return results;

However, as soon as I add the action to a presentation property, the workflow spends an eternity "processing presentation". The action itself works quickly enough. I added it to a test workflow as part of its schema and I get the desired array of strings back instantly. It's only when it's being used for presentation that it's giving me a problem.

Anyone have any ideas what's going on?