QIBASE driver incorrect work with FireBird database, who contain text strings in Win-1251 codepage

I work with FireBird database, who contain text strings in Win-1251 codepage. I use QT4.6(4.7) and QIBASE driver. QSqlQuery returned square instead cyrillic simbols.
The string

db.setConnectOptions("ISC_DPB_LC_CTYPE=WIN-1251");

don't work.
...
According to QIBASE Unicode Support and Text Encoding [doc.qt.nokia.com] it falls back to Unicode when the encoding you request is not available (not availabel in QTextCodec, according to a quick look to the source code of the qsql_ibase plugin).

CP1251 and Windows-1251 are valid in QTextCodec but may not be correct for IBase; on the other hand WIN1251 and WIN-1251 may be correct for IBase but are not valid for QTextCodec. So this seems to be a loose-loose situation.
...
see that topic
http://developer.qt.nokia.com/forums/viewthread/1910/