diff --git a/GPy/kern/periodic_Matern32.py b/GPy/kern/periodic_Matern32.py
index 3e81044c..eeadf1c8 100644
--- a/GPy/kern/periodic_Matern32.py
+++ b/GPy/kern/periodic_Matern32.py
@@ -21,13 +21,14 @@ class periodic_Matern32(kernpart):
     """
 
     def __init__(self,D=1,variance=1.,lengthscale=None,period=2*np.pi,n_freq=10,lower=0.,upper=4*np.pi):
-        assert D==1
+        assert D==1, "Periodic kernels are only defined for D=1"
         self.name = 'periodic_Mat32'
         self.D = D
         if lengthscale is not None:
-            assert lengthscale.shape==(self.D,)
+            lengthscale = np.asarray(lengthscale)
+            assert lengthscale.size == 1, "Wrong size: only one lengthscale needed"
         else:
-            lengthscale = np.ones(self.D)
+            lengthscale = np.ones(1)
         self.lower,self.upper = lower, upper
         self.Nparam = 3
         self.n_freq = n_freq
diff --git a/GPy/kern/periodic_Matern52.py b/GPy/kern/periodic_Matern52.py
index abdd6a7d..2db3d223 100644
--- a/GPy/kern/periodic_Matern52.py
+++ b/GPy/kern/periodic_Matern52.py
@@ -21,13 +21,14 @@ class periodic_Matern52(kernpart):
     """
 
     def __init__(self,D=1,variance=1.,lengthscale=None,period=2*np.pi,n_freq=10,lower=0.,upper=4*np.pi):
-        assert D==1
+        assert D==1, "Periodic kernels are only defined for D=1"
         self.name = 'periodic_Mat52'
         self.D = D
         if lengthscale is not None:
-            assert lengthscale.shape==(self.D,)
+            lengthscale = np.asarray(lengthscale)
+            assert lengthscale.size == 1, "Wrong size: only one lengthscale needed"
         else:
-            lengthscale = np.ones(self.D)
+            lengthscale = np.ones(1)
         self.lower,self.upper = lower, upper
         self.Nparam = 3
         self.n_freq = n_freq
diff --git a/GPy/kern/periodic_exponential.py b/GPy/kern/periodic_exponential.py
index 2637afe5..d99bada8 100644
--- a/GPy/kern/periodic_exponential.py
+++ b/GPy/kern/periodic_exponential.py
@@ -21,13 +21,14 @@ class periodic_exponential(kernpart):
     """
 
     def __init__(self,D=1,variance=1.,lengthscale=None,period=2*np.pi,n_freq=10,lower=0.,upper=4*np.pi):
-        assert D==1
+        assert D==1, "Periodic kernels are only defined for D=1"
         self.name = 'periodic_exp'
         self.D = D
         if lengthscale is not None:
-            assert lengthscale.shape==(self.D,)
+            lengthscale = np.asarray(lengthscale)
+            assert lengthscale.size == 1, "Wrong size: only one lengthscale needed"
         else:
-            lengthscale = np.ones(self.D)
+            lengthscale = np.ones(1)
         self.lower,self.upper = lower, upper
         self.Nparam = 3
         self.n_freq = n_freq
@@ -45,7 +46,7 @@ class periodic_exponential(kernpart):
         r =  np.sqrt(r1**2 + r2**2)
         psi = np.where(r1 != 0, (np.arctan(r2/r1) + (r1<0.)*np.pi),np.arcsin(r2))
         return r,omega[:,0:1], psi
-    
+
     def _int_computation(self,r1,omega1,phi1,r2,omega2,phi2):
         Gint1 = 1./(omega1+omega2.T)*( np.sin((omega1+omega2.T)*self.upper+phi1+phi2.T) - np.sin((omega1+omega2.T)*self.lower+phi1+phi2.T)) + 1./(omega1-omega2.T)*( np.sin((omega1-omega2.T)*self.upper+phi1-phi2.T) - np.sin((omega1-omega2.T)*self.lower+phi1-phi2.T) )
         Gint2 = 1./(omega1+omega2.T)*( np.sin((omega1+omega2.T)*self.upper+phi1+phi2.T) - np.sin((omega1+omega2.T)*self.lower+phi1+phi2.T)) +  np.cos(phi1-phi2.T)*(self.upper-self.lower)