HTTP Post nested array

I am currently trying to POST data to an API and it seems that there is a problem with POST’ing an array within the JSON object. I am declaring the object as follows:

let data = {
      "deliveryStreet": this.formGroup.get('shippingLineOne').value,
      "deliveryBuilding": this.formGroup.get('shippingLineTwo').value,
      "deliveryCity": this.formGroup.get('shippingCity').value,
      "deliveryProvince": this.formGroup.get('shippingProvince').value,
      "deliveryPostalCode": this.formGroup.get('shippingPostalCode').value,
      "cardName": this.formGroup.get('cardholderName').value,
      "cardNumber": this.formGroup.get('ccNum').value,
      "cardExpiry": this.formGroup.get('ccExp').value,
      "cardCvv": this.formGroup.get('cardCvv').value,
      "orderItems": this.cart.cartItems
    };

Where this.cart.cartItems is an array of objects. When I try and POST the data it would appear that there is malformation of the orderItems, I have tried using JSON.stringify on both the entire object and just the array itself with no effect. Does anyone have an idea why this nested array won’t POST to an endpoint?