I've demonstrated the problem here:
$ pie() { echo $1; }; pie '*'
1 2 3 4 5 file
Expanded. Bother.
But the solution is quite simple. You just need to quote it but in a way that bash will understand. We use double-quotes. It will be replaced with the variable contents but it won't be expanded.
$ pie() { echo "$1"; }; pie '*'
*
