diff --git a/src/progressbar/progressbar.ts b/src/progressbar/progressbar.ts
index c29003769b..57ab5cd30d 100644
--- a/src/progressbar/progressbar.ts
+++ b/src/progressbar/progressbar.ts
@@ -21,6 +21,8 @@ import {NgbProgressbarConfig} from './progressbar-config';
 export class NgbProgressbar {
   /**
    * The maximal value to be displayed in the progressbar.
+   *
+   * Should be a positive number.
    */
   @Input() max: number;