An "Invalid get_string() identifier" exception occur (and included in the exported file) when certain additional user profile fields (e.g. skype) are included in the grade export.

How to reproduce:

1. Set debug level to 'DEVELOPR' and turn display debug messages on
2. Add 'skype' to the list of the 'Grade export user profile fields' setting (grade_export_userprofilefields)
3. Export grades of a course, regardless of file type and if there are users in the course or not
4. View the exported file
5. EXPECTED: well-formed grade export including a 'skype' column. ACTUALLY HAPPENS: HTML snipped with an "Invalid get_string() identifier" exception at the beginning of the file; the actual column is named '[[skype]]'