bash script error stty: standard input: Inappropriate ioctl for device

I'm using here-documents in a bash script to automate installation and setup where a password is required many times. I enter the password once and the script passes it to the various commands. In most instances the here-document approach handles this fine. However, in one case I get this error:

Enter VNC password: stty: standard input: Inappropriate ioctl for deviceVerify password:    stty: standard input: Inappropriate ioctl for device

Please notice that this error message is from x11vnc -storepassword (not from sudo.)

My problem is related to x11vnc -storepasswd and here's my code:

sudo x11vnc -storepasswd ~/.vnc/passwd << ENDDOCpasswordpasswordyENDDOC

That obviously (from the error) does not work. I would appreciate a working example of how to implement sudo x11vnc -storepasswd ~/.vnc/passwd in a script.

In case it helps, the prompts look like this:

Enter VNC password:
Verify password:
Write password to /home/user/.vnc/passwd? [y]/n n

Will using expect be a better solution? If so, how would I use it in this case? (I have never used expect before but I have looked at a lot of examples since posting this question and I cannot get expect to work on my own.)