diff --git a/django/db/models/fields/json.py b/django/db/models/fields/json.py
index 29c6ff09260..1b0601018ed 100644
--- a/django/db/models/fields/json.py
+++ b/django/db/models/fields/json.py
@@ -366,14 +366,25 @@ class CaseInsensitiveMixin:
 class KeyTransformIsNull(lookups.IsNull):
     # key__isnull=False is the same as has_key='key'
     def as_oracle(self, compiler, connection):
+        sql, params = HasKey(
+            self.lhs.lhs,
+            self.lhs.key_name,
+        ).as_oracle(compiler, connection)
         if not self.rhs:
-            return HasKey(self.lhs.lhs, self.lhs.key_name).as_oracle(compiler, connection)
-        return super().as_sql(compiler, connection)
+            return sql, params
+        # Column doesn't have a key or IS NULL.
+        lhs, lhs_params, _ = self.lhs.preprocess_lhs(compiler, connection)
+        return '(NOT %s OR %s IS NULL)' % (sql, lhs), tuple(params) + tuple(lhs_params)
 
     def as_sqlite(self, compiler, connection):
+        template = 'JSON_TYPE(%s, %%s) IS NULL'
         if not self.rhs:
-            return HasKey(self.lhs.lhs, self.lhs.key_name).as_sqlite(compiler, connection)
-        return super().as_sql(compiler, connection)
+            template = 'JSON_TYPE(%s, %%s) IS NOT NULL'
+        return HasKey(self.lhs.lhs, self.lhs.key_name).as_sql(
+            compiler,
+            connection,
+            template=template,
+        )
 
 
 class KeyTransformIn(lookups.In):
diff --git a/docs/releases/3.1.5.txt b/docs/releases/3.1.5.txt
index 4a7fcc58764..0d111184461 100644
--- a/docs/releases/3.1.5.txt
+++ b/docs/releases/3.1.5.txt
@@ -9,4 +9,6 @@ Django 3.1.5 fixes several bugs in 3.1.4.
 Bugfixes
 ========
 
-* ...
+* Fixed ``__isnull=True`` lookup on key transforms for
+  :class:`~django.db.models.JSONField` with Oracle and SQLite
+  (:ticket:`32252`).
diff --git a/tests/model_fields/test_jsonfield.py b/tests/model_fields/test_jsonfield.py
index f71efcff616..c6b2f85e1ef 100644
--- a/tests/model_fields/test_jsonfield.py
+++ b/tests/model_fields/test_jsonfield.py
@@ -586,6 +586,10 @@ class TestQuerying(TestCase):
             NullableJSONModel.objects.filter(value__a__isnull=True),
             self.objs[:3] + self.objs[5:],
         )
+        self.assertSequenceEqual(
+            NullableJSONModel.objects.filter(value__j__isnull=True),
+            self.objs[:4] + self.objs[5:],
+        )
         self.assertSequenceEqual(
             NullableJSONModel.objects.filter(value__a__isnull=False),
             [self.objs[3], self.objs[4]],