diff --git a/openmc/filter.py b/openmc/filter.py
index 6e389ba2d..f9d93d91a 100644
--- a/openmc/filter.py
+++ b/openmc/filter.py
@@ -750,7 +750,7 @@ class MeshFilter(Filter):
         cv.check_type('filter mesh', mesh, openmc.MeshBase)
         self._mesh = mesh
         if isinstance(mesh, openmc.UnstructuredMesh):
-            self.bins = [ (n, 1, 1) for n in range(1, mesh.volumes.size + 1) ]
+            self.bins = [(n, 1, 1) for n in range(1, len(mesh.volumes) + 1)]
         else:
             self.bins = list(mesh.indices)
 
diff --git a/openmc/mesh.py b/openmc/mesh.py
index 8b199fa25..1117116e1 100644
--- a/openmc/mesh.py
+++ b/openmc/mesh.py
@@ -616,6 +616,7 @@ class UnstructuredMesh(MeshBase):
     def __init__(self, mesh_id=None, name='', filename=''):
         super().__init__(mesh_id, name)
         self._filename = filename
+        self._volumes = []
 
     @property
     def filename(self):