diff --git a/.gitignore b/.gitignore
index fa78fbc..ed2d659 100644
--- a/.gitignore
+++ b/.gitignore
@@ -161,3 +161,5 @@ bayesnewton.egg-info/*
 .idea/
 
 src/data/audio_training_textures_Heavy_rain_on_hard_surface.wav
+
+build/
diff --git a/bayesnewton/likelihoods.py b/bayesnewton/likelihoods.py
index f4b7c32..6848e09 100644
--- a/bayesnewton/likelihoods.py
+++ b/bayesnewton/likelihoods.py
@@ -171,7 +171,7 @@ def gauss_newton(self, y, f):
         log_target = -0.5 * V.T @ V + self.log_normaliser(f)
         jacobian = -J.T @ V + ZJ
         # hessian_approx = -J.T @ J + self.log_normaliser_hessian(f)
-        hessian_approx = -J.T @ J - np.diag((ZJ**2).flattent())
+        hessian_approx = -J.T @ J - np.diag((ZJ**2).flatten())
         # second_order_term = -H.T * V
         return log_target, jacobian, hessian_approx  # , second_order_term