Expect Script For Sftp Fails Under Cron

The following expect script works perfectly from the command line, but fails under cron:

Code:
#!/usr/bin/expect
spawn sftp -b /root/sftp.txt root@***.***.***.***
expect "password:"
send "password
"
interact
The output is:

Code:
spawn sftp -b /root/sftp.txt root@***.***.***.***
root@***.***.***.***'s password:
I can't work out if the password just isn't getting sent, or if the interact command is causing the script to fail. I also can't fathom why this would only cause a problem under cron.