[Python] seaborn heatmap cell style for nan-Values

I have created a heatmap with seaborn in a jupyter notebook:

import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns

sns.set(rc={"figure.figsize": (10, 10), })
sns.set(font_scale=0.9)
ax = plt.subplots()
ax = sns.heatmap(
forHeatmap,
square=True, # make cells square
cbar_kws={'fraction' : 0.01, 'label': 'Länge (km)'}, # shrink colour bar
cmap='OrRd', # use orange/red colour map
linewidth=0.5 # space between cells
)


And the output (without label on the axis) looks like this:

Unfortunately I cannot show the underlying data, but it is a pandas dataframe with the index set on the variable s How can I cange the style (not just the color!) for the Nan-values? I would like to have them as grey squares, just as the squares representing values.

EDIT: When I do the same with small testdata, it looks like this:

import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns

data = pd.DataFrame(
{"x": (1, 2, 3, 4), "y": (1, 2, np.nan, 4), "z": (14, 15, 23, 2)}
).set_index("x")



The dataframe looks now like it, with the index set on the first column (x):



Now, I plot with this:

sns.set(
rc={
"figure.figsize": (5, 5),
}
)
sns.set(font_scale=0.9)
ax = plt.subplots()
ax = sns.heatmap(
data,
square=True, # make cells square
cmap="OrRd", # use orange/red colour map
linewidth=0.5, # space between cells
)



And the result is:



As shown, the cell for the nan-value is represented as 4 small squares instead of one. How can I change it? As I couldn't find a thread regarding this topic I think it may be jupyter notebook related?

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