title: Adding new column to GeoDataFrame and it says values' length does not match index when it does
body: <p>The goal is to read a shapefile, get the area of each polygon (municipalities), and add the area in a new column named 'Area'.</p>
  
  <pre><code>cs = gpd.read_file('munis.shp')
  area_fd = []
  area = cs['geometry'].map(lambda p: p.area / 10**6)
  area_fd.append(area)
  cs = cs.assign(Area =  area_fd) #or cs['Area'] = area_fd
  </code></pre>
  
  <p>When I run <code>print(cs['NAME_2'].head)</code> it returns the 38 munis (0-38) and <code>print(area_fd)</code> gives 38 values too. But when I run the code, it says <code>ValueError: Length of values does not match length of index</code>. I already looked up to similar GISSE and SO threads but no solution worked.</p>