diff --git a/stdimage/management/commands/rendervariations.py b/stdimage/management/commands/rendervariations.py
index a2ab078..eab3390 100644
--- a/stdimage/management/commands/rendervariations.py
+++ b/stdimage/management/commands/rendervariations.py
@@ -25,11 +25,8 @@ def add_arguments(self, parser):
                             help='Replace existing files.')
 
     def handle(self, *args, **options):
-        replace = options.get('replace')
-        if len(options['field_path']):
-            routes = options['field_path']
-        else:
-            routes = [options['field_path']]
+        replace = options.get('replace', False)
+        routes = options.get('field_path', [])
         for route in routes:
             try:
                 app_label, model_name, field_name = route.rsplit('.')
@@ -73,7 +70,7 @@ def render(field, images, count, replace, do_render):
             ' ', progressbar.Bar(),
         )) as bar:
             with ProcessPoolExecutor() as executor:
-                while next(executor.map(render_field_variations, kwargs_list)):
+                for _ in executor.map(render_field_variations, kwargs_list):
                     bar += 1
 
 
diff --git a/tests/test_commands.py b/tests/test_commands.py
index a45c492..2578c1b 100644
--- a/tests/test_commands.py
+++ b/tests/test_commands.py
@@ -1,6 +1,7 @@
 import hashlib
 import os
 import time
+from concurrent.futures import ThreadPoolExecutor
 
 import pytest
 from django.core.management import CommandError, call_command
@@ -11,7 +12,16 @@
 
 
 @pytest.mark.django_db
-class TestRenderVariations(object):
+class TestRenderVariations:
+
+    @pytest.fixture(autouse=True)
+    def _swap_concurrent_executor(self, monkeypatch):
+        """Use ThreadPoolExecutor for coverage reports."""
+        monkeypatch.setattr(
+            'concurrent.futures.ProcessPoolExecutor',
+            ThreadPoolExecutor,
+        )
+
     def test_no_options(self, image_upload_file):
         obj = ThumbnailModel.objects.create(
             image=image_upload_file
diff --git a/tests/test_models.py b/tests/test_models.py
index eec35b9..21b30d5 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -12,7 +12,7 @@
 from django.core.files.uploadedfile import SimpleUploadedFile
 
 
-class UUID4Monkey(object):
+class UUID4Monkey:
     hex = '653d1c6863404b9689b75fa930c9d0a0'
 
 
@@ -38,7 +38,7 @@ class UUID4Monkey(object):
 ]
 
 
-class TestStdImage(object):
+class TestStdImage:
     fixtures = {}
 
     @pytest.fixture(autouse=True)
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 78b5c36..38c1672 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -9,7 +9,7 @@
 
 
 @pytest.mark.django_db
-class TestRenderVariations(object):
+class TestRenderVariations:
     def test_render_variations(self, image_upload_file):
         instance = ManualVariationsModel.customer_manager.create(
             image=image_upload_file
@@ -31,7 +31,7 @@ def test_render_variations(self, image_upload_file):
         assert os.path.exists(path)
 
 
-class TestUploadTo(object):
+class TestUploadTo:
     def test_file_name(self):
         file_name = UploadTo()(object(), '/path/to/file.jpeg')
         assert file_name == '/path/to/file.jpeg'