WLSTException: Error occurred while performing startEdit

I am getting this exception while trying to modify the 'Sever Start' configurations in WLS 12.2.1.3:

WLSTException: Error occurred while performing startEdit : Cannot call Edit functions when you are not in the Edit tree.

Here is the script snippet:

connect(...)

cd('/Servers')

servers = cmo.getServers()

for server in servers:

 thisServer = server.getName()

 msServerName=server.getName()

 print msServerName

 cd('/Servers/' + msServerName + '/ServerStart/' + msServerName)

 startEdit()

 ...

 save()

activate(block='true')