How can I limit the input only to yes or no

Question

I have written the following script and found out that the enter button has the same function as entering "No". How can I make it, that the enter button has no effect on the script ?

weiter_mit_func()
{
printf "Script fortsetzen (y/n): "
read -n 1  ANTWORT
if [ "x$ANTWORT" = "xY" -o "x$ANTWORT" = "xy" ]
then
 echo "Info: Aktion wird weitergeführt!"
else
 echo "WARNING: Aktion vom Benutzer abgebrochen!"
 exit 100
 export GLOBAL_ERROR=100
 unset OPT_LISTE
fi
}


weiter_mit_func