ncl画特定等值线报错。。。

RDSHQEASUIAYRIW

写的命令:
res@cnLevelSelectionMode="ExplicitLevels"
res@cnLevels=(/50,100,200/)

报错：
fatal:Dimension size of attribute and right-hand side of assignment do not match
有大神知道为什么会出现这种问题吗{:eb303:}{:eb303:}