SIL - cannot get values from multi user picker custom field

[Chris Shaw]

For a transition post function I'm trying to retrieve and use the user id values added into a multi-user picker custom field. I set a string array equal to the field values:

string [] values = #{my multi user picker field};

Then I try to set the value of a single line text field to the first index of the array:

string first_value = arrayGetElement( values, 1);

#(my text field} = first_value;

When I try to run the transition, I just get the working spinning wheel and then it gives up and nothing happens. I am not sure if the first index is 0 or 1, but there are 3 values so should work. Are the individual values stored in the user picker just strings, or a special user type? I see them as user ids when you first enter them, but then shown as {Last Name, First Name} when viewing the issue.