diff --git a/angular-translate/angular-translate.d.ts b/angular-translate/angular-translate.d.ts
index b191bf455..141804791 100644
--- a/angular-translate/angular-translate.d.ts
+++ b/angular-translate/angular-translate.d.ts
@@ -27,7 +27,7 @@ declare module angular.translate {
     }
 
     interface IPartialLoader<T> {
-        addPart(name : string, priority : number) : T;
+        addPart(name : string, priority? : number) : T;
         setPart(lang : string, part : string, table : ITranslationTable) : T;
         deletePart(name : string) : T;
         isPartAvailable(name : string) : boolean;