set_printquality bug in Excel.PageSetup - DOTNET

I think I discovered a bug in set_printquality method in
Excel.PageSetup. Example:
....
Excel.Sheets xlsheets = wb.Sheets;
Excel.Worksheet excelWorksheet = (Excel.Worksheet)xlsheets[1];
Excel.PageSetup ps = excelWorksheet.PageSetup;
object x = ps.get_PrintQuality(1);

---> debug shows: x = 600.0 (the value from PrintQuality field in
Excel's "Page Setup" form)
---> debug shows: ps.CenterFooter = ""

x = 300.0;
ps.set_PrintQuality(x, Missing.Value);

---> debug shows: ps.CenterFooter = "300"

After saving the file, the PrintQuality field is not changed, but you
get a nice footer with value 300.

Is this a bug to report, or am I doing something wrong here?