diff --git a/django_mongodb_backend/operations.py b/django_mongodb_backend/operations.py
index cb1e93d..dcdf7fb 100644
--- a/django_mongodb_backend/operations.py
+++ b/django_mongodb_backend/operations.py
@@ -147,7 +147,9 @@ class DatabaseOperations(BaseDatabaseOperations):
         Convert dict data to a string so that JSONField.from_db_value() can
         decode it using json.loads().
         """
-        return json.dumps(value)
+        target = getattr(expression, 'target', None)
+        encoder = target.encoder if target else None
+        return json.dumps(value, cls=encoder)
 
     def convert_timefield_value(self, value, expression, connection):
         if value is not None:
diff --git a/tests/model_fields_/models.py b/tests/model_fields_/models.py
index b25b94a..b8fa3c0 100644
--- a/tests/model_fields_/models.py
+++ b/tests/model_fields_/models.py
@@ -6,6 +6,25 @@ from django_mongodb_backend.fields import ArrayField, EmbeddedModelField, Object
 from django_mongodb_backend.models import EmbeddedModel
 
 
+import json
+from bson import json_util
+
+
+class BSONEncoder(json.JSONEncoder):
+    def encode(self, obj):
+        return json_util.dumps(obj)
+
+
+class BSONDecoder(json.JSONDecoder):
+    def decode(self, obj):
+        return json_util.loads(obj)
+
+
+# JSONField
+class JSONModel(models.Model):
+    value = models.JSONField(encoder=BSONEncoder, decoder=BSONDecoder)
+
+
 # ObjectIdField
 class ObjectIdModel(models.Model):
     field = ObjectIdField()
diff --git a/tests/model_fields_/test_jsonfield.py b/tests/model_fields_/test_jsonfield.py
new file mode 100644
index 0000000..a860b7b
--- /dev/null
+++ b/tests/model_fields_/test_jsonfield.py
@@ -0,0 +1,15 @@
+from django.db.models import JSONField
+from django.test import TestCase
+from bson import ObjectId
+
+from .models import JSONModel
+
+
+class TestSaveLoad(TestCase):
+    def test_null(self):
+        object_id = ObjectId()
+        value = {"key": object_id}
+        obj = JSONModel(value=value)
+        obj.save()
+        obj.refresh_from_db()
+        self.assertEqual(obj.value, value)