diff --git a/SimPEG/DiffOperators.py b/SimPEG/DiffOperators.py
index 86fd35f7..b2a7a6a5 100644
--- a/SimPEG/DiffOperators.py
+++ b/SimPEG/DiffOperators.py
@@ -286,7 +286,7 @@ class DiffOperators(object):
     _nodalVectorAve = None
     nodalVectorAve = property(**nodalVectorAve())
 
-    def getMass(self, loc='e', materialProp=None, inv=False):
+    def getMass(self, materialProp=None, loc='e', inv=False):
         """ Produces mass matricies.
 
         Kwargs: