运行以下代码时，报错：
ZeroDivisionError: float division by zero
代码如下：
import matplotlib
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
from matplotlib.colors import ListedColormap
import numpy as np
from PIL import Image
fig=plt.figure()
ax1=fig.add_axes([0.1,0.1,0.8,0.8])
map=Basemap(projection='cass',llcrnrlon=-180,llcrnrlat=-90,urcrnrlon=180,urcrnrlat=90,lat_0=0,lon_0=0.,ax=ax1,resolution=None)
map.shadedrelief(scale=0.5)
plt.show()
求如何画全的浮雕图，示例只画了一部分，要画经度-180到180，维度-90到90的
