No actually!

It was a functuion that returned the upper case version of a string.

Why not use UPPER I hear you cry! Well, I did. I just wanted to see if using a user-defined function would work just as well. It didn't.

The function accepted a VARCHAR2 string and had a body...

RETURN (UPPER(string));

Works fine in SQLPlus using

SELECT UPPER_FUNC('lowercase') FROM dual;

Returns> LOWERCASE

But when creating the index based on this function I get an error.