diff --git a/openmc/deplete/__init__.py b/openmc/deplete/__init__.py
index 32588588ef..08338c3f74 100644
--- a/openmc/deplete/__init__.py
+++ b/openmc/deplete/__init__.py
@@ -5,16 +5,17 @@ openmc.deplete
 A depletion front-end tool.
 """
 
+from unittest import Mock
+
 from .dummy_comm import DummyCommunicator
 try:
     from mpi4py import MPI
     comm = MPI.COMM_WORLD
     have_mpi = True
-    mpi_sum = MPI.SUM
 except ImportError:
     comm = DummyCommunicator()
     have_mpi = False
-    mpi_sum = lambda x: x
+    MPI = Mock()
 
 from .nuclide import *
 from .chain import *
diff --git a/openmc/deplete/results.py b/openmc/deplete/results.py
index 8c47706b75..bdee89b6ca 100644
--- a/openmc/deplete/results.py
+++ b/openmc/deplete/results.py
@@ -10,7 +10,7 @@ from warnings import warn
 import numpy as np
 import h5py
 
-from . import comm, have_mpi, mpi_sum
+from . import comm, have_mpi, MPI
 from .reaction_rates import ReactionRates
 
 _VERSION_RESULTS = (1, 0)
@@ -452,7 +452,7 @@ class Results(object):
         results.power = power
         results.proc_time = proc_time
         if results.proc_time is not None:
-            results.proc_time = comm.reduce(proc_time, op=mpi_sum)
+            results.proc_time = comm.reduce(proc_time, op=MPI.SUM)
 
         results.export_to_hdf5("depletion_results.h5", step_ind)