diff --git a/ckeditor/ckeditor-tests.ts b/ckeditor/ckeditor-tests.ts
index bcf2a7807b..97a5c73937 100644
--- a/ckeditor/ckeditor-tests.ts
+++ b/ckeditor/ckeditor-tests.ts
@@ -37,6 +37,23 @@ function test_CKEDITOR() {
     CKEDITOR.replaceAll((textarea, config) => false);
 }
 
+function test_config() {
+    var config1: CKEDITOR.config = {
+        toolbar: 'basic',
+    };
+    var config2: CKEDITOR.config = {
+        toolbar: [
+            [ 'mode', 'document', 'doctools' ],
+            [ 'clipboard', 'undo' ],
+            '/',
+            [ 'find', 'selection', 'spellchecker' ],
+            [ 'basicstyles', 'cleanup' ],
+            '/',
+            [ 'list', 'indent', 'blocks', 'align', 'bidi' ],
+        ],
+    };
+}
+
 function test_dom_comment() {
     var type = CKEDITOR.NODE_COMMENT;
     var nativeNode = document.createComment('Example');
diff --git a/ckeditor/ckeditor.d.ts b/ckeditor/ckeditor.d.ts
index b5009fa031..d43446dd83 100644
--- a/ckeditor/ckeditor.d.ts
+++ b/ckeditor/ckeditor.d.ts
@@ -807,7 +807,7 @@ declare namespace CKEDITOR {
         templates_files?: Object;
         templates_replaceContent?: boolean;
         title?: string | boolean;
-        toolbar?: string | (string[])[];
+        toolbar?: string | (string | string[])[];
         toolbarCanCollapse?: boolean;
         toolbarGroupCycling?: boolean;
         toolbarGroups?: toolbarGroups[];