Issue plotting coordinates using Basemap

[kiton]

Having extracted geo coordinates from Twitter JSON, I created a new pandas data frame:

    df3
    Out[140]:
    lat lon pos
    63 -26.298376 28.070223 (-26.29837597, 28.07022309)
    116 1.487180 124.861180 (1.48718, 124.86118)
    199 33.708842 -84.294066 (33.70884247, -84.29406577)
    349 35.368816 -96.955488 (35.36881649, -96.95548791)
    379 14.830463 120.284751 (14.83046326, 120.2847507)
    859 35.151530 -80.727347 (35.15153029, -80.72734744)

Now, my goal is to plot the coordinates on the map using Basemap:

    import matplotlib.pyplot as plt
    from mpl_toolkits.basemap import Basemap
     
    # Set map size
    fig, ax = plt.subplots(figsize=(10,20))

    # Set USA coordinates
    m = Basemap(resolution='c', # c, l, i, h, f or None
            projection='merc',
            lat_0=39, lon_0=-99,  ## center of the map
            llcrnrlon=-125, llcrnrlat= 25, urcrnrlon=-65, urcrnrlat=49)

    # Read county boundaries
    shp_info = m.readshapefile('cb_2015_us_county_500k',
                                   'counties',
                                   drawbounds=True)

    # Set geo coordinates to be plotted 
    lats = df3['lat'] [:]  #all values from column lat
    lons = df3['lon'] [:]  #all values from column lon
    x,y = m(lats, lons)

    # Place points on the map
    m.plot(x, y, 'ro', markersize=5)

    m.bluemarble()
    plt.show()

However, for some reason I get this error:

Error:
    TypeError                                 Traceback (most recent call last)
    TypeError: expected a writable bytes-like object

    The above exception was the direct cause of the following exception:

    SystemError                               Traceback (most recent call last)
    <ipython-input-144-d258e6080d9d> in <module>()
         18 lats = df3['lat'] [:]
         19 lons = df3['lon'] [:]
    ---> 20 x,y = m(lats, lons)
         21 
         22 # place it on the map

    /Users/mymac/anaconda/lib/python3.6/site-packages/mpl_toolkits/basemap/__init__.py in __call__(self, x, y, inverse)
       1146             except TypeError:
       1147                 y = [_dg2rad*yy for yy in y]
    -> 1148         xout,yout = self.projtran(x,y,inverse=inverse)
       1149         if self.celestial and inverse:
       1150             try:

    /Users/mymac/anaconda/lib/python3.6/site-packages/mpl_toolkits/basemap/proj.py in __call__(self, *args, **kw)
        284             outxy = self._proj4(xy, inverse=inverse)
        285         else:
    --> 286             outx,outy = self._proj4(x, y, inverse=inverse)
        287         if inverse:
        288             if self.projection in ['merc','mill','gall']:

    /Users/mymac/anaconda/lib/python3.6/site-  packages/mpl_toolkits/basemap/pyproj.py in __call__(self, *args, **kw)
        386             _proj.Proj._inv(self, inx, iny, radians=radians, errcheck=errcheck)
        387         else:
    --> 388             _proj.Proj._fwd(self, inx, iny, radians=radians, errcheck=errcheck)
        389         # if inputs were lists, tuples or floats, convert back.
        390         outx = _convertback(xisfloat,xislist,xistuple,inx)

    _proj.pyx in _proj.Proj._fwd (src/_proj.c:1571)()

    SystemError: <class 'RuntimeError'> returned a result with an error set

Can anybody please tell me where I am making a mistake.