using applescript reply all

I'm trying to automate a simple reply all to a selected email from the inbox, but for some reason, I can't add new content to the reply email.

The console seems to say that the content is being "set", but nothing appears in the generated email (just the old reply to content). Am I doing something wrong here?

Thanks in advance!

tell application "Mail"

set theSelection to selection

set theMessage to item 1 of theSelection

try

set theNewMessage to replytheMessage with opening window and reply to all

tell theNewMessage

set content of theNewMessage to "###enter message content here###"

end tell

end try

end tell