diff --git a/torchdyn/numerics/solvers/ode.py b/torchdyn/numerics/solvers/ode.py
index 7406fb9..aa066c6 100644
--- a/torchdyn/numerics/solvers/ode.py
+++ b/torchdyn/numerics/solvers/ode.py
@@ -144,7 +144,7 @@ def __init__(self, dtype=torch.float32):
 
     def step(self, f, x, t, dt, k1=None, args=None) -> Tuple:
         c, a, bsol, berr = self.tableau
-        if k1 == None: k1 = f(t, x)
+        if k1 is None: k1 = f(t, x)
         k2 = f(t + c[0] * dt, x + dt * a[0] * k1)
         k3 = f(t + c[1] * dt, x + dt * (a[1][0] * k1 + a[1][1] * k2))
         k4 = f(t + c[2] * dt, x + dt * a[2][0] * k1 + dt * a[2][1] * k2 + dt * a[2][2] * k3)