diff --git a/lib/galaxy/model/__init__.py b/lib/galaxy/model/__init__.py
index 2d1c0508422..caa5790eb89 100644
--- a/lib/galaxy/model/__init__.py
+++ b/lib/galaxy/model/__init__.py
@@ -1619,13 +1619,11 @@ class Dataset( object ):
     def get_total_size( self ):
         if self.total_size is not None:
             return self.total_size
-        if self.file_size:
-            # for backwards compatibility, set if unset
-            self.set_total_size()
-            db_session = object_session( self )
-            db_session.flush()
-            return self.total_size
-        return 0
+        # for backwards compatibility, set if unset
+        self.set_total_size()
+        db_session = object_session( self )
+        db_session.flush()
+        return self.total_size
 
     def set_total_size( self ):
         if self.file_size is None:
diff --git a/scripts/cleanup_datasets/cleanup_datasets.py b/scripts/cleanup_datasets/cleanup_datasets.py
index 59b33e6fe0b..e189ef11ca2 100755
--- a/scripts/cleanup_datasets/cleanup_datasets.py
+++ b/scripts/cleanup_datasets/cleanup_datasets.py
@@ -483,7 +483,7 @@ def _purge_dataset( app, dataset, remove_from_disk, info_only=False ):
                                 if hda.history.user is not None and hda.history.user not in usage_users:
                                     usage_users.append( hda.history.user )
                         for user in usage_users:
-                            user.total_disk_usage -= dataset.total_size
+                            user.total_disk_usage -= dataset.get_total_size()
                             app.sa_session.add( user )
                     print "Purging dataset id", dataset.id
                     dataset.purged = True
diff --git a/test/unit/test_galaxy_mapping.py b/test/unit/test_galaxy_mapping.py
index 69a96011077..f12d1368811 100644
--- a/test/unit/test_galaxy_mapping.py
+++ b/test/unit/test_galaxy_mapping.py
@@ -410,7 +410,7 @@ class MappingTests( unittest.TestCase ):
     @classmethod
     def setUpClass(cls):
         # Start the database and connect the mapping
-        cls.model = mapping.init( "/tmp", "sqlite:///:memory:", create_tables=True )
+        cls.model = mapping.init( "/tmp", "sqlite:///:memory:", create_tables=True, object_store=MockObjectStore() )
         assert cls.model.engine is not None
 
     @classmethod
@@ -439,6 +439,21 @@ class MappingTests( unittest.TestCase ):
         cls.model.session.expunge_all()
 
 
+class MockObjectStore(object):
+
+    def __init__(self):
+        pass
+
+    def size(self, dataset):
+        return 42
+
+    def exists(self, *args, **kwds):
+        return True
+
+    def get_filename(self, *args, **kwds):
+        return "dataest_14.dat"
+
+
 def get_suite():
     suite = unittest.TestSuite()
     suite.addTest( MappingTests( "test_basic" ) )