diff --git a/GPy/likelihoods/likelihood.py b/GPy/likelihoods/likelihood.py
index 33698eb2..813f912f 100644
--- a/GPy/likelihoods/likelihood.py
+++ b/GPy/likelihoods/likelihood.py
@@ -203,7 +203,7 @@ class Likelihood(Parameterized):
 
     def _conditional_mean(self, f):
         """Quadrature calculation of the conditional mean: E(Y_star|f)"""
-        raise NotImplementedError, "implement this function to make predictions"
+        raise NotImplementedError("implement this function to make predictions")
 
     def predictive_variance(self, mu,variance, predictive_mean=None, Y_metadata=None):
         """
diff --git a/GPy/likelihoods/link_functions.py b/GPy/likelihoods/link_functions.py
index a4ddc760..60e260e7 100644
--- a/GPy/likelihoods/link_functions.py
+++ b/GPy/likelihoods/link_functions.py
@@ -182,7 +182,7 @@ class Heaviside(GPTransformation):
         return np.where(f>0, 1, 0)
 
     def dtransf_df(self,f):
-        raise NotImplementedError, "This function is not differentiable!"
+        raise NotImplementedError("This function is not differentiable!")
 
     def d2transf_df2(self,f):
-        raise NotImplementedError, "This function is not differentiable!"
+        raise NotImplementedError("This function is not differentiable!")