If enemyButton is a MovieClip (created via attachMovie, maybe) and not strongly typed as a EnemyButton class, then the ID property becomes dynamic. In this situation, if your list of names contains incorrect data (missing ID field, maybe), then the ID property will remain undefined on some instances of the MovieClip.
You can check the list of data used to generate movie clips. You can run into the same error if you have blank lines in your data.
This has nothing to do with event listeners.