diff --git a/django/db/migrations/state.py b/django/db/migrations/state.py
index 088f5c41f0..6e023415bd 100644
--- a/django/db/migrations/state.py
+++ b/django/db/migrations/state.py
@@ -91,10 +91,11 @@ class ProjectState:
     def __init__(self, models=None, real_apps=None):
         self.models = models or {}
         # Apps to include from main registry, usually unmigrated ones
-        if real_apps:
-            self.real_apps = real_apps if isinstance(real_apps, set) else set(real_apps)
+        if real_apps is None:
+            real_apps = set()
         else:
-            self.real_apps = set()
+            assert isinstance(real_apps, set)
+        self.real_apps = real_apps
         self.is_delayed = False
         # {remote_model_key: {model_key: [(field_name, field)]}}
         self.relations = None
diff --git a/docs/releases/4.0.txt b/docs/releases/4.0.txt
index 0fa87b2810..24ce7285e0 100644
--- a/docs/releases/4.0.txt
+++ b/docs/releases/4.0.txt
@@ -544,6 +544,10 @@ Miscellaneous
 * The undocumented ``TRANSLATOR_COMMENT_MARK`` constant is moved from
   ``django.template.base`` to ``django.utils.translation.template``.
 
+* The ``real_apps`` argument of the undocumented
+  ``django.db.migrations.state.ProjectState.__init__()`` method must now be a
+  set if provided.
+
 .. _deprecated-features-4.0:
 
 Features deprecated in 4.0
diff --git a/tests/migrations/test_state.py b/tests/migrations/test_state.py
index 01ffc48802..5be29ca2f1 100644
--- a/tests/migrations/test_state.py
+++ b/tests/migrations/test_state.py
@@ -924,6 +924,10 @@ class StateTests(SimpleTestCase):
             1,
         )
 
+    def test_real_apps_non_set(self):
+        with self.assertRaises(AssertionError):
+            ProjectState(real_apps=['contenttypes'])
+
     def test_ignore_order_wrt(self):
         """
         Makes sure ProjectState doesn't include OrderWrt fields when