here, repository_id_list is already a list.

stack/erp5.cfg

@@ -275,7 +275,7 @@ initialization =
   os.environ['SOFTWARE_HOME'] = os.path.abspath(os.path.dirname(os.path.dirname(Zope2.__file__)))
   os.environ['ZOPE_SCRIPTS'] = ''
   repository_id_list = list(reversed('''${erp5_repository_list:repository_id_list}'''.split()))
-  sys.path[0:0] = ['/'.join(['''${buildout:parts-directory}''', x]) for x in repository_id_list.split()]
+  sys.path[0:0] = ['/'.join(['''${buildout:parts-directory}''', x]) for x in repository_id_list]
 
 [instance-recipe-egg]
 recipe = zc.recipe.egg