diff --git a/src/plugins/data/server/ui_settings.ts b/src/plugins/data/server/ui_settings.ts
index 7b386e864fbc1..5ab638a087783 100644
--- a/src/plugins/data/server/ui_settings.ts
+++ b/src/plugins/data/server/ui_settings.ts
@@ -168,12 +168,12 @@ export function getUiSettings(
       name: i18n.translate('data.advancedSettings.defaultIndexTitle', {
         defaultMessage: 'Default data view',
       }),
-      value: null,
+      value: '',
       type: 'string',
       description: i18n.translate('data.advancedSettings.defaultIndexText', {
         defaultMessage: 'Used by discover and visualizations when a data view is not set.',
       }),
-      schema: schema.nullable(schema.string()),
+      schema: schema.string(),
     },
     [UI_SETTINGS.COURIER_IGNORE_FILTER_IF_FIELD_NOT_IN_INDEX]: {
       name: i18n.translate('data.advancedSettings.courier.ignoreFilterTitle', {
diff --git a/test/api_integration/apis/data_views/default_index_pattern/default_index_pattern.ts b/test/api_integration/apis/data_views/default_index_pattern/default_index_pattern.ts
index d34ad5ccd5f4d..b980c684e529b 100644
--- a/test/api_integration/apis/data_views/default_index_pattern/default_index_pattern.ts
+++ b/test/api_integration/apis/data_views/default_index_pattern/default_index_pattern.ts
@@ -48,7 +48,7 @@ export default function ({ getService }: FtrProviderContext) {
           expect(response5.status).to.be(200);
 
           const response6 = await supertest.get(defaultPath);
-          expect(response6.body[serviceKeyId]).to.be(null);
+          expect(response6.body[serviceKeyId]).to.be('');
         });
       });
     });
diff --git a/x-pack/test_serverless/api_integration/test_suites/common/data_views/default_index_pattern/default_index_pattern.ts b/x-pack/test_serverless/api_integration/test_suites/common/data_views/default_index_pattern/default_index_pattern.ts
index 0ac0bec2250ed..067c24d605e6c 100644
--- a/x-pack/test_serverless/api_integration/test_suites/common/data_views/default_index_pattern/default_index_pattern.ts
+++ b/x-pack/test_serverless/api_integration/test_suites/common/data_views/default_index_pattern/default_index_pattern.ts
@@ -72,7 +72,7 @@ export default function ({ getService }: FtrProviderContext) {
           // TODO: The response comes back undefined in Serverless
           const body = response6.body[serviceKeyId];
           const expected = body === undefined ? null : body;
-          expect(expected).to.be(null);
+          expect(expected).to.be('');
         });
       });
     });