Is this xpath valid ? Scrapy not letting me use it

I'm trying to use this xpath in scrapy, but scrapy says that it's an invalid syntax. When I use the command $x('...') in google debug , I get a correct output.

//*[@id="popUp_PC_tbl_Data"]/tbody/tr[2]/td[2]/span

When i change "popUp_PC_tbl_Data" to 'popUp_PC_tbl_Data', I get no errors. However my .csv file is empty.