To avoid this and other similar problems, just pass the string indirectly (ie not hardcode it into the identifier). There a few ways to do that:
1) Put the string in a variable:
//var %a = , | echo -a $asc(%a)
2) Make an alias:
/alias ascii echo -a $asc($$1)
then type /ascii ,
3) In this example with $asc() you can also use $? or $input():
//echo -a $asc($$input(Enter character:,e))