Problems with seaborn (ndim) - python-3.x

Question

I am using seaborn to plot a very simple data set. Here is what I do:
import seaborn as sns
import pandas as pd
df = pd.read_excel('myfile.xlsx')
sns.set(style="white")
g = sns.PairGrid(df, diag_sharey=False)
g.map_lower(sns.kdeplot)
g.map_upper(sns.scatterplot)
g.map_diag(sns.kdeplot, lw=3)
I get the following error: AttributeError: 'NoneType' object has no attribute 'ndim'. Weirdly, the plot is ploted in parts (see below).
Any idea why that is the case and what I can do to solve the issue?
EDIT:
The dataframe has the following attributes:
plan_change int64
user_login float64
new_act_ratio float64
on_time int64
Unfortunately, I cannot upload the data set. However I can say, that plotting other seaborn graphs works just fine.
The total error message is the following:
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-16-2dbc61abd2bd> in <module>()
3 g = sns.PairGrid(df, diag_sharey=False)
4 g.map_lower(sns.kdeplot)
----> 5 g.map_upper(sns.scatterplot)
6 g.map_diag(sns.kdeplot, lw=3)
7
/anaconda/lib/python3.5/site-packages/seaborn/axisgrid.py in map_upper(self, func, **kwargs)
1488 color = self.palette[k] if kw_color is None else kw_color
1489 func(data_k[x_var], data_k[y_var], label=label_k,
-> 1490 color=color, **kwargs)
1491
1492 self._clean_axis(ax)
/anaconda/lib/python3.5/site-packages/seaborn/relational.py in scatterplot(x, y, hue, style, size, data, palette, hue_order, hue_norm, sizes, size_order, size_norm, markers, style_order, x_bins, y_bins, units, estimator, ci, n_boot, alpha, x_jitter, y_jitter, legend, ax, **kwargs)
1333 x_bins=x_bins, y_bins=y_bins,
1334 estimator=estimator, ci=ci, n_boot=n_boot,
-> 1335 alpha=alpha, x_jitter=x_jitter, y_jitter=y_jitter, legend=legend,
1336 )
1337
/anaconda/lib/python3.5/site-packages/seaborn/relational.py in __init__(self, x, y, hue, size, style, data, palette, hue_order, hue_norm, sizes, size_order, size_norm, dashes, markers, style_order, x_bins, y_bins, units, estimator, ci, n_boot, alpha, x_jitter, y_jitter, legend)
850
851 plot_data = self.establish_variables(
--> 852 x, y, hue, size, style, units, data
853 )
854
/anaconda/lib/python3.5/site-packages/seaborn/relational.py in establish_variables(self, x, y, hue, size, style, units, data)
155 units=units
156 )
--> 157 plot_data = pd.DataFrame(plot_data)
158
159 # Option 3:
/anaconda/lib/python3.5/site-packages/pandas/core/frame.py in __init__(self, data, index, columns, dtype, copy)
264 dtype=dtype, copy=copy)
265 elif isinstance(data, dict):
--> 266 mgr = self._init_dict(data, index, columns, dtype=dtype)
267 elif isinstance(data, ma.MaskedArray):
268 import numpy.ma.mrecords as mrecords
/anaconda/lib/python3.5/site-packages/pandas/core/frame.py in _init_dict(self, data, index, columns, dtype)
400 arrays = [data[k] for k in keys]
401
--> 402 return _arrays_to_mgr(arrays, data_names, index, columns, dtype=dtype)
403
404 def _init_ndarray(self, values, index, columns, dtype=None, copy=False):
/anaconda/lib/python3.5/site-packages/pandas/core/frame.py in _arrays_to_mgr(arrays, arr_names, index, columns, dtype)
5382
5383 # don't force copy because getting jammed in an ndarray anyway
-> 5384 arrays = _homogenize(arrays, index, dtype)
5385
5386 # from BlockManager perspective
/anaconda/lib/python3.5/site-packages/pandas/core/frame.py in _homogenize(data, index, dtype)
5693 v = lib.fast_multiget(v, oindex.values, default=NA)
5694 v = _sanitize_array(v, index, dtype=dtype, copy=False,
-> 5695 raise_cast_failure=False)
5696
5697 homogenized.append(v)
/anaconda/lib/python3.5/site-packages/pandas/core/series.py in _sanitize_array(data, index, dtype, copy, raise_cast_failure)
2917
2918 # scalar like
-> 2919 if subarr.ndim == 0:
2920 if isinstance(data, list): # pragma: no cover
2921 subarr = np.array(data, dtype=object)
AttributeError: 'NoneType' object has no attribute 'ndim'