Fix "Can't modify frozen string" error

Change-Id: I7eb2b66ed08bde3fc703358a86e578ff115db54a
Reviewed-on: http://review.couchbase.org/26979
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>