diff --git a/GPy/kern/src/stationary.py b/GPy/kern/src/stationary.py
index c10384bb..79d4b954 100644
--- a/GPy/kern/src/stationary.py
+++ b/GPy/kern/src/stationary.py
@@ -135,7 +135,7 @@ class Stationary(Kern):
             #X2, = self._slice_X(X2)
             X1sq = np.sum(np.square(X),1)
             X2sq = np.sum(np.square(X2),1)
-            r2 = -2.*np.dot(X, X2.T) + X1sq[:,None] + X2sq[None,:]
+            r2 = -2.*np.dot(X, X2.T) + (X1sq[:,None] + X2sq[None,:])
             r2 = np.clip(r2, 0, np.inf)
             return np.sqrt(r2)