Passing variable as index in property, Groovy

Hi.  I am having trouble getting a property to expand in Groovy using the right index after getting a list of data from a data source. I'm trying to pass the index to the property as a variable in the script but it won't work. The values for the random index looks good in the log, and the property expands on any valid number when using a number for the index.

For example (1), this produces a good result:
result = context.expand( '${DataSourceStationNumber#STATION_NUM::9}')

This example (2) fails even thought the random number variable is good and valid:
result = context.expand( '${DataSourceStationNumber#STATION_NUM::randomIndex}')

result is blank in example 2. Example 1 works with any valid number. 
What's the syntax for this, or is it possibly a type issue in the randomIndex? (E.g., needs to be cast as string?)
Thanks.