Ionic “track by” error with tinder cards

I started learning Ionic 2 weeks ago and right now I am creating a small project to practice.You can download my project from my Github :

I am trying to use tinder like cards which is created from the ionic team itself:

I have tried every possible solution with “track by” expression but I am still getting this error:

Error: [ngRepeat:dupes] Duplicates in a repeater are not allowed. Use ‘track by’ expression to specify unique keys. Repeater: card in cards track by, Duplicate key: undefined, Duplicate value: undefined

I have tried to fix this error with “track by $index” and "track by "(every card has an “id” attribute in .Json file) but it didn’t worked. I guess that i made a small mistake and can’t find it.

Can someone please help me? Thanks