diff --git a/django/contrib/admin/util.py b/django/contrib/admin/util.py
index a85045c515..07013d1d4b 100644
--- a/django/contrib/admin/util.py
+++ b/django/contrib/admin/util.py
@@ -88,8 +88,7 @@ def flatten_fieldsets(fieldsets):
     field_names = []
     for name, opts in fieldsets:
         for field in opts['fields']:
-            # type checking feels dirty, but it seems like the best way here
-            if type(field) == tuple:
+            if isinstance(field, (list, tuple)):
                 field_names.extend(field)
             else:
                 field_names.append(field)
diff --git a/tests/regressiontests/admin_util/tests.py b/tests/regressiontests/admin_util/tests.py
index e9e122a9f0..274103cf87 100644
--- a/tests/regressiontests/admin_util/tests.py
+++ b/tests/regressiontests/admin_util/tests.py
@@ -5,8 +5,8 @@ from datetime import datetime
 from django.conf import settings
 from django.contrib import admin
 from django.contrib.admin import helpers
-from django.contrib.admin.util import (display_for_field, label_for_field,
-    lookup_field, NestedObjects)
+from django.contrib.admin.util import (display_for_field, flatten_fieldsets,
+    label_for_field, lookup_field, NestedObjects)
 from django.contrib.admin.views.main import EMPTY_CHANGELIST_VALUE
 from django.contrib.sites.models import Site
 from django.db import models, DEFAULT_DB_ALIAS
@@ -300,3 +300,21 @@ class UtilTests(unittest.TestCase):
                          '<label for="id_text" class="required inline">&amp;text:</label>')
         self.assertEqual(helpers.AdminField(form, 'cb', is_first=False).label_tag(),
                          '<label for="id_cb" class="vCheckboxLabel required inline">&amp;cb</label>')
+
+    def test_flatten_fieldsets(self):
+        """
+        Regression test for #18051
+        """
+        fieldsets = (
+            (None, {
+                'fields': ('url', 'title', ('content', 'sites'))
+            }),
+        )
+        self.assertEqual(flatten_fieldsets(fieldsets), ['url', 'title', 'content', 'sites'])
+
+        fieldsets = (
+            (None, {
+                'fields': ['url', 'title', ['content', 'sites'])
+            }),
+        )
+        self.assertEqual(flatten_fieldsets(fieldsets), ['url', 'title', 'content', 'sites'])