Plotstyle Classic creates fill opacity = None, Error in Latex

[Phillipp33]

Error appears when using plotstyle Classic. Could be with others as well, but I didn't check all possible entries.

import torch
import torch.nn.functional as F
import numpy as np
import matplotlib.pyplot as plt
import tikzplotlib

# Used for tikzplotlib
plt.style.use("classic")

# Create x values
x = torch.linspace(-3.5, 3.5, 100)
x_np = x.numpy()

# Compute activations and derivatives
sigmoid = torch.sigmoid(x).numpy()
sigmoid_prime = sigmoid * (1 - sigmoid)

tanh = torch.tanh(x).numpy()
tanh_prime = 1 - torch.tanh(x)**2

# Create subplots
fig, axs = plt.subplots(1, 2, figsize=(10, 4))

# Plot Sigmoid
axs[0].plot(x_np, sigmoid, label='f')
axs[0].plot(x_np, sigmoid_prime, label="f'")
axs[0].set_title('Sigmoid')
axs[0].legend(loc='upper left')

# Plot Tanh
axs[1].plot(x_np, tanh, label='f')
axs[1].plot(x_np, tanh_prime, label="f'")
axs[1].set_title('Tanh')
axs[1].legend(loc='upper left')

# Adjust layout
plt.tight_layout()
tikzplotlib.save("Classic_activations.tex")

# Show plot
plt.show()

This outputs a .tex file with fill opacity = None, which is not allowed in Latex.
Manually changing this entry to something between 0 and 1 removes the error.