在等值线图上叠加地形出错

DABEIGE

程序：
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load"$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
f1=addfile("C:/Users/Administrator/Desktop/data/temp.1987.nc","r")
ff1=short2flt(f1->t(1:5,:,:))

g1=addfile("C:/Users/Administrator/Desktop/data/temp.1987.nc","r")
gg1=short2flt(g1->t(6:10,:,:))

t=ff1(0,:,:)
t=gg1(0,:,:)-ff1(0,:,:)
t1=smth9_Wrap(t,0.50,-0.25,True)
shpfn2 = "C:/Users/Administrator/Desktop/data/tibetan.shp"
wks=gsn_open_wks("png","C:/Users/Administrator/Desktop/chenxu/contour")
res = True
res@gsnDraw = False
res@gsnFrame = False
res@pmTickMarkDisplayMode = "Always"
res@gsnAddCyclic=False
res@mpMaxLatF = 50
res@mpMinLatF = 0
res@mpMinLonF = 50
res@mpMaxLonF = 120

res@cnLineThicknessF = 3.0
res@gsnContourNegLineDashPattern = 1
res@cnSmoothingOn        = True
res@cnLineLabelPlacementMode="Constant"
plot = gsn_csm_contour_map(wks,t1,res)


plres = True
plres@gsLineColor = "black"
plres@gsLineThickness = "3.0"
shp_plot = gsn_add_shapfile_polylines(wks,plot,shpfn2,plres)
报错信息

地形文件应该是没有问题的，不知道为啥会有这个报错