NPOI won't allow me to open Excel 2007 files

Question

So here's the code:

            using (FileStream file = new FileStream(databaseLocation, FileMode.Open, FileAccess.Read))
        {
            // _Sheet is of type IWorksheet
            _Sheet = new XSSFWorkbook(file).GetSheetAt(0);
        }

This, continuously and regardless of what overload I use, throws a variety of exceptions. I've chosen the particular overload above for this question because everybody else seems to be able to open Excel 2007 (.xlsx) files with (basically) this exact method!

This particular overload provides me with the error "ZipException - EOF in header". Another lovingly descriptive error "ZipException - Cannot find central dictionary" is given for the string overload (i.e. _Sheet = new XSSFWorkbook(databaseLocation).GetSheetAt(0);).

I don't know if it's an issue with the way that NPOI implements the ICSharpCode's SharpZipLib. Either way, I need to be able to open Excel 2007 files.

I have tried resetting file.Position to 0, amongst other things suggested on a Google search, but there isn't much to go off of. Help me before I rip my hair out!