diff --git a/django/db/models/lookups.py b/django/db/models/lookups.py
index d0b30ace1d..8428d38d65 100644
--- a/django/db/models/lookups.py
+++ b/django/db/models/lookups.py
@@ -92,6 +92,10 @@ class Transform(RegisterLookupMixin):
             bilateral_transforms.append((self.__class__, self.init_lookups))
         return bilateral_transforms
 
+    @cached_property
+    def contains_aggregate(self):
+        return self.lhs.contains_aggregate
+
 
 class Lookup(RegisterLookupMixin):
     lookup_name = None
@@ -194,6 +198,10 @@ class Lookup(RegisterLookupMixin):
     def as_sql(self, compiler, connection):
         raise NotImplementedError
 
+    @cached_property
+    def contains_aggregate(self):
+        return self.lhs.contains_aggregate or getattr(self.rhs, 'contains_aggregate', False)
+
 
 class BuiltinLookup(Lookup):
     def process_lhs(self, compiler, connection, lhs=None):
diff --git a/django/db/models/sql/where.py b/django/db/models/sql/where.py
index 2ba6ceac33..8cad3df9a5 100644
--- a/django/db/models/sql/where.py
+++ b/django/db/models/sql/where.py
@@ -315,9 +315,9 @@ class WhereNode(tree.Node):
 
     @classmethod
     def _contains_aggregate(cls, obj):
-        if not isinstance(obj, tree.Node):
-            return getattr(obj.lhs, 'contains_aggregate', False) or getattr(obj.rhs, 'contains_aggregate', False)
-        return any(cls._contains_aggregate(c) for c in obj.children)
+        if isinstance(obj, tree.Node):
+            return any(cls._contains_aggregate(c) for c in obj.children)
+        return obj.contains_aggregate
 
     @cached_property
     def contains_aggregate(self):
@@ -336,6 +336,7 @@ class EverythingNode(object):
     """
     A node that matches everything.
     """
+    contains_aggregate = False
 
     def as_sql(self, compiler=None, connection=None):
         return '', []
@@ -345,11 +346,16 @@ class NothingNode(object):
     """
     A node that matches nothing.
     """
+    contains_aggregate = False
+
     def as_sql(self, compiler=None, connection=None):
         raise EmptyResultSet
 
 
 class ExtraWhere(object):
+    # The contents are a black box - assume no aggregates are used.
+    contains_aggregate = False
+
     def __init__(self, sqls, params):
         self.sqls = sqls
         self.params = params
@@ -410,6 +416,10 @@ class Constraint(object):
 
 
 class SubqueryConstraint(object):
+    # Even if aggregates would be used in a subquery, the outer query isn't
+    # interested about those.
+    contains_aggregate = False
+
     def __init__(self, alias, columns, targets, query_object):
         self.alias = alias
         self.columns = columns
diff --git a/docs/releases/1.8.9.txt b/docs/releases/1.8.9.txt
index 080d6506e8..8c5445b549 100644
--- a/docs/releases/1.8.9.txt
+++ b/docs/releases/1.8.9.txt
@@ -23,3 +23,6 @@ Bugfixes
   ``db_index=True`` or ``unique=True`` to a ``CharField`` or ``TextField`` that
   already had the other specified, or when removing one of them from a field
   that had both (:ticket:`26034`).
+
+* Fixed a crash when using an ``__in`` lookup inside a ``Case`` expression
+  (:ticket:`26071`).
diff --git a/tests/expressions_case/tests.py b/tests/expressions_case/tests.py
index 27aef931c0..de1859b368 100644
--- a/tests/expressions_case/tests.py
+++ b/tests/expressions_case/tests.py
@@ -8,7 +8,7 @@ from uuid import UUID
 
 from django.core.exceptions import FieldError
 from django.db import connection, models
-from django.db.models import F, Q, Max, Min, Value
+from django.db.models import F, Q, Max, Min, Sum, Value
 from django.db.models.expressions import Case, When
 from django.test import TestCase
 from django.utils import six
@@ -119,6 +119,17 @@ class CaseExpressionTests(TestCase):
             transform=attrgetter('integer', 'join_test')
         )
 
+    def test_annotate_with_in_clause(self):
+        fk_rels = FKCaseTestModel.objects.filter(integer__in=[5])
+        self.assertQuerysetEqual(
+            CaseTestModel.objects.only('pk', 'integer').annotate(in_test=Sum(Case(
+                When(fk_rel__in=fk_rels, then=F('fk_rel__integer')),
+                default=Value(0),
+            ))).order_by('pk'),
+            [(1, 0), (2, 0), (3, 0), (2, 0), (3, 0), (3, 0), (4, 5)],
+            transform=attrgetter('integer', 'in_test')
+        )
+
     def test_annotate_with_join_in_condition(self):
         self.assertQuerysetEqual(
             CaseTestModel.objects.annotate(join_test=Case(