Using spreadsheet gem with Oracle data

I have successfully used the spreadsheet gem to export data out of a
mysql database. I am now trying to do something similar using an Oracle
11g database configured for UTF-8 data. But I get the following error
on the
‘workbook =’ line of code:

Iconv::InvalidEncoding in VisitLogsController#index

invalid encoding ('UTF-8, ‘UTF-8’)

Has anybody seen this?? Thanks!