diff --git a/pina/model/spline.py b/pina/model/spline.py
index 2c5aa6e..2328986 100644
--- a/pina/model/spline.py
+++ b/pina/model/spline.py
@@ -148,19 +148,19 @@ class Spline(torch.nn.Module):
 
         self._knots = value
 
-    def forward(self, x_):
+    def forward(self, x):
         """
         Forward pass of the spline model.
 
-        :param torch.Tensor x_: points to be evaluated.
-        :return: the spline evaluated at x_
+        :param torch.Tensor x: points to be evaluated.
+        :return: the spline evaluated at x
         :rtype: torch.Tensor
         """
         t = self.knots
         k = self.k
         c = self.control_points
 
-        basis = map(lambda i: self.basis(x_, k, i, t)[:, None], range(len(c)))
+        basis = map(lambda i: self.basis(x, k, i, t)[:, None], range(len(c)))
         y = (torch.cat(list(basis), dim=1) * c).sum(axis=1)
 
         return y
diff --git a/setup.py b/setup.py
index b1909a6..8c7b9ac 100644
--- a/setup.py
+++ b/setup.py
@@ -20,7 +20,7 @@ REQUIRED = [
 
 EXTRAS = {
     'docs': [
-        'sphinx',
+        'sphinx>5.0',
         'sphinx_rtd_theme',
         'sphinx_copybutton',
         'sphinx_design',