diff --git a/GPy/models/sparse_GP_regression.py b/GPy/models/sparse_GP_regression.py
index 78a3f9a8..a9bb0206 100644
--- a/GPy/models/sparse_GP_regression.py
+++ b/GPy/models/sparse_GP_regression.py
@@ -142,6 +142,9 @@ class sparse_GP_regression(GP_regression):
         return dL_dtheta
 
     def dL_dZ(self):
+        """
+        The derivative of the bound wrt the inducing inputs Z
+        """
         #re-cast computations in psi2 back to psi1:
         dL_dpsi1 = self.dL_dpsi1 + 2.*np.dot(self.dL_dpsi2,self.psi1)