# django__django-11905
# Loki Mode Multi-Agent Patch
# Attempts: 1

--- a/django/db/models/lookups.py
+++ b/django/db/models/lookups.py
@@ -1,3 +1,4 @@
+import warnings
 from datetime import datetime, timezone
 
 from django.core.exceptions import EmptyResultSet
@@ -5,6 +6,7 @@
 from django.db.models.sql.query import Query
 from django.db.models.sql.where import NothingNode, SubqueryConstraint, WhereNode
 from django.utils import formats
+from django.utils.deprecation import RemovedInDjango40Warning
 from django.utils.functional import cached_property
 
 
@@ -463,6 +465,13 @@ class IsNull(BuiltinLookup):
     prepare_rhs = False
 
     def as_sql(self, compiler, connection):
+        if not isinstance(self.rhs, bool):
+            warnings.warn(
+                'The QuerySet value for an isnull lookup must be True or '
+                'False.',
+                RemovedInDjango40Warning,
+            )
         sql, params = compiler.compile(self.lhs)
         if self.rhs:
             return "%s IS NULL" % sql, params