diff --git a/pina/equation/equation_factory.py b/pina/equation/equation_factory.py
index 057ea65d4..da9c55647 100644
--- a/pina/equation/equation_factory.py
+++ b/pina/equation/equation_factory.py
@@ -239,19 +239,19 @@ def equation(input_, output_):
                 )
 
             # Ensure consistency of c length
-            if len(self.c) != (len(input_lbl) - 1) and len(self.c) > 1:
+            if self.c.shape[-1] != len(input_lbl) - 1 and self.c.shape[-1] > 1:
                 raise ValueError(
                     "If 'c' is passed as a list, its length must be equal to "
                     "the number of spatial dimensions."
                 )
 
             # Repeat c to ensure consistent shape for advection
-            self.c = self.c.repeat(output_.shape[0], 1)
-            if self.c.shape[1] != (len(input_lbl) - 1):
-                self.c = self.c.repeat(1, len(input_lbl) - 1)
+            c = self.c.repeat(output_.shape[0], 1)
+            if c.shape[1] != (len(input_lbl) - 1):
+                c = c.repeat(1, len(input_lbl) - 1)
 
             # Add a dimension to c for the following operations
-            self.c = self.c.unsqueeze(-1)
+            c = c.unsqueeze(-1)
 
             # Compute the time derivative and the spatial gradient
             time_der = grad(output_, input_, components=None, d="t")
@@ -262,7 +262,7 @@ def equation(input_, output_):
             tmp = tmp.transpose(-1, -2)
 
             # Compute advection term
-            adv = (tmp * self.c).sum(dim=tmp.tensor.ndim - 2)
+            adv = (tmp * c).sum(dim=tmp.tensor.ndim - 2)
 
             return time_der + adv
 
diff --git a/pina/problem/zoo/helmholtz.py b/pina/problem/zoo/helmholtz.py
index 5f3f956af..0f38780c7 100644
--- a/pina/problem/zoo/helmholtz.py
+++ b/pina/problem/zoo/helmholtz.py
@@ -48,11 +48,10 @@ def __init__(self, alpha=3.0):
         :type alpha: float | int
         """
         super().__init__()
-
-        self.alpha = alpha
         check_consistency(alpha, (int, float))
+        self.alpha = alpha
 
-        def forcing_term(self, input_):
+        def forcing_term(input_):
             """
             Implementation of the forcing term.
             """
diff --git a/tests/test_equation/test_equation_factory.py b/tests/test_equation/test_equation_factory.py
index 4a9875115..be01427cb 100644
--- a/tests/test_equation/test_equation_factory.py
+++ b/tests/test_equation/test_equation_factory.py
@@ -104,7 +104,7 @@ def test_advection_equation(c):
 
     # Should fail if c is a list and its length != spatial dimension
     with pytest.raises(ValueError):
-        Advection([1, 2, 3])
+        equation = Advection([1, 2, 3])
         residual = equation.residual(pts, u)