diff --git a/tests/.testing_harness.py.swp b/tests/.testing_harness.py.swp
new file mode 100644
index 0000000000..72fea4fb0b
Binary files /dev/null and b/tests/.testing_harness.py.swp differ
diff --git a/tests/test_source_file/test_source_file.py b/tests/test_source_file/test_source_file.py
index af998ee339..cc146e5c9f 100644
--- a/tests/test_source_file/test_source_file.py
+++ b/tests/test_source_file/test_source_file.py
@@ -67,7 +67,7 @@ class SourceFileTestHarness(TestHarness):
     def _run_openmc_restart(self):
         # Get the number of MPI processes.
         if self._opts.mpi_exec:
-            mpi_procs = self._opts.mpi_np
+            mpi_procs = int(self._opts.mpi_np)
         else:
             mpi_procs = 1
 
diff --git a/tests/test_statepoint_restart/test_statepoint_restart.py b/tests/test_statepoint_restart/test_statepoint_restart.py
index c62eb67df4..8d960128dd 100644
--- a/tests/test_statepoint_restart/test_statepoint_restart.py
+++ b/tests/test_statepoint_restart/test_statepoint_restart.py
@@ -26,7 +26,7 @@ class StatepointRestartTestHarness(TestHarness):
     def _run_openmc_restart(self):
         # Get the number of MPI processes.
         if self._opts.mpi_exec:
-            mpi_procs = self._opts.mpi_np
+            mpi_procs = int(self._opts.mpi_np)
         else:
             mpi_procs = 1
 
diff --git a/tests/testing_harness.py b/tests/testing_harness.py
index 99f2917d80..997052cfd0 100644
--- a/tests/testing_harness.py
+++ b/tests/testing_harness.py
@@ -45,7 +45,7 @@ class TestHarness(object):
 
     def _run_openmc(self):
         if self._opts.mpi_exec:
-            mpi_procs = self._opts.mpi_np
+            mpi_procs = int(self._opts.mpi_np)
         else:
             mpi_procs = 1