diff --git a/GPy/likelihoods/noise_models/gp_transformations.py b/GPy/likelihoods/noise_models/gp_transformations.py
index 41cc5790..73e08cac 100644
--- a/GPy/likelihoods/noise_models/gp_transformations.py
+++ b/GPy/likelihoods/noise_models/gp_transformations.py
@@ -118,7 +118,7 @@ class Step(GPTransformation):
     """
     def transf(self,f):
         #transformation goes here
-        return np.where(f>0, 1, 0)
+        return np.where(f>0, 1, -1)
 
     def dtransf_df(self,f):
         pass