diff --git a/moment/moment-tests.ts b/moment/moment-tests.ts
index 056620ada..535c13597 100644
--- a/moment/moment-tests.ts
+++ b/moment/moment-tests.ts
@@ -132,6 +132,7 @@ a8.diff(b8, 'years')
 a8.diff(b8, 'years', true);
 
 moment([2007, 0, 29]).toDate();
+moment([2007, 1, 23]).toISOString();
 moment(1318874398806).valueOf(); 
 moment(1318874398806).unix();
 moment([2000]).isLeapYear();
diff --git a/moment/moment.d.ts b/moment/moment.d.ts
index 6a46739ba..e9748f57f 100644
--- a/moment/moment.d.ts
+++ b/moment/moment.d.ts
@@ -105,6 +105,7 @@ interface Moment {
     diff(b: Moment, soort: string, round: boolean): number;
 
     toDate(): Date;
+    toISOString(): string;
     unix(): number;
 
     isLeapYear(): boolean;