diff --git a/src/modal/modal-config.spec.ts b/src/modal/modal-config.spec.ts
new file mode 100644
index 0000000000..046f9e8a78
--- /dev/null
+++ b/src/modal/modal-config.spec.ts
@@ -0,0 +1,21 @@
+import {inject} from '@angular/core/testing';
+
+import {NgbModalConfig} from './modal-config';
+
+describe('NgbModalConfig', () => {
+
+  it('should have sensible default values', inject([NgbModalConfig], (config: NgbModalConfig) => {
+
+       expect(config.ariaLabelledBy).toBeUndefined();
+       expect(config.backdrop).toBe(true);
+       expect(config.backdropClass).toBeUndefined();
+       expect(config.beforeDismiss).toBeUndefined();
+       expect(config.centered).toBeUndefined();
+       expect(config.container).toBeUndefined();
+       expect(config.injector).toBeUndefined();
+       expect(config.keyboard).toBe(true);
+       expect(config.scrollable).toBeUndefined();
+       expect(config.size).toBeUndefined();
+       expect(config.windowClass).toBeUndefined();
+     }));
+});
diff --git a/src/modal/modal-config.ts b/src/modal/modal-config.ts
index f0339a63d5..cde3f8ab37 100644
--- a/src/modal/modal-config.ts
+++ b/src/modal/modal-config.ts
@@ -94,7 +94,16 @@ export interface NgbModalOptions {
 * @since 3.1.0
 */
 @Injectable({providedIn: 'root'})
-export class NgbModalConfig implements NgbModalOptions {
+export class NgbModalConfig implements Required<NgbModalOptions> {
+  ariaLabelledBy: string;
   backdrop: boolean | 'static' = true;
+  beforeDismiss: () => boolean | Promise<boolean>;
+  centered: boolean;
+  container: string;
+  injector: Injector;
   keyboard = true;
+  scrollable: boolean;
+  size: 'sm' | 'lg' | 'xl';
+  windowClass: string;
+  backdropClass: string;
 }