DBMS_XMLSCHEMA.generateschema gives ORA-22303: type not found

I just had someone call about this.
SELECT DBMS_XMLSCHEMA.generateschema('your_schema','your_type') FROM DUAL;
gives ORA-22303: type "your_schema"."your_type" not found
ora-21700
ora-06512

Just upper case your_schema and your_type -- YOUR_SCHEMA and YOUR_TYPE.

Hope this helps.