我用stata13执行下面命令； xi: reg $y  $x  i.year  i.id时出现了下列的提示错误：

no room to add more variables
    Up to 5,000 variables are currently allowed, although you could reset the maximum using set maxvar; see help
    memory.
r(900);
end of do-file
r(900);


请教大家，这该怎么解决呢？谢谢！