diff --git a/SimPEG/Regularization.py b/SimPEG/Regularization.py
index 195e6203..7fe976fc 100644
--- a/SimPEG/Regularization.py
+++ b/SimPEG/Regularization.py
@@ -311,6 +311,9 @@ class BaseRegularization(object):
             tmp = indActive
             indActive = np.zeros(mesh.nC, dtype=bool)
             indActive[tmp] = True
+        if indActive is not None and mapping is None:
+            mapping = Maps.IdentityMap(nP=indActive.nonzero()[0].size)
+
         self.regmesh = RegularizationMesh(mesh,indActive)
         self.mapping = mapping or self.mapPair(mesh)
         self.mapping._assertMatchesPair(self.mapPair)
diff --git a/tests/base/test_regularization.py b/tests/base/test_regularization.py
index 5aa21ad5..6dddfb74 100644
--- a/tests/base/test_regularization.py
+++ b/tests/base/test_regularization.py
@@ -65,10 +65,8 @@ class RegularizationTests(unittest.TestCase):
                     elif mesh.dim == 3:
                         indActive = Utils.mkvc(mesh.gridCC[:,-1] <= 2*np.sin(2*np.pi*mesh.gridCC[:,0])+0.5 * 2*np.sin(2*np.pi*mesh.gridCC[:,1])+0.5)
 
-                    mapping = Maps.IdentityMap(nP=indActive.nonzero()[0].size)
-
                     for indAct in [indActive, indActive.nonzero()[0]]: # test both bool and integers
-                        reg = r(mesh, mapping=mapping, indActive=indAct)
+                        reg = r(mesh, indActive=indAct)
                         m = np.random.rand(mesh.nC)[indAct]
                         reg.mref = np.ones_like(m)*np.mean(m)