cartCreate only creates an empty cart

[AlanForsVispero]

I'm using version 2021-10 (release candidate) of the api to create a shopping cart. The cart is created, but it's always empty. If I intentionally set the merchandiseId to an invalid value, it correctly reports a user error for an invalid merchandiseId. When I use a valid merchandiseId, the cart is created but contains an empty lines array.

My query:

mutation ($id: ID!, $auth: String!, $email: String!) {
  cartCreate(input: {
    lines: [
      {
        merchandiseId: $id, 
        quantity: 1, 
        attributes: [
          {
            key: "AuthenticationCode", 
            value: $auth
          }
        ]
      }
    ], 
    buyerIdentity: {
      email: $email
    }
  }) 
  {
    cart {
      id
      checkoutUrl
      lines(first: 5) {
        edges {
          node {
            id
            merchandise {
              ... on ProductVariant {
                id
              }
            }
          }
        }
      }
    }
    userErrors {
      code
      field
      message
    }
  }
}

And the query variables:

{
  "id":"<variantId>",
  "auth":"F54596DE5C661DBDB8A4",
  "email":"afors@vispero.com"
}

I get the response:

{
  "data": {
    "cartCreate": {
      "cart": {
        "id": "Z2lkOi8vc2hvcGlmeS9DYXJ0L2RiNmJiNGFhZjYyZDg1YjMxZTJkYmY2YTgxZTY4MDJj",
        "checkoutUrl": "https:\/\/fs-recurring-billing-sandbox.myshopify.com\/cart\/c\/db6bb4aaf62d85b31e2dbf6a81e6802c",
        "lines": {
          "edges": []
        }
      },
      "userErrors": []
    }
  }
}

Can anyone see a problem with this? I've tried everything I can think of.

[AlanForsVispero]

I decided to check for an empty cart after creation, and if it is empty, then use a cartLinesAdd mutation to add the line item. Same result. The query succeeds, but the cart is still empty.

[AlanForsVispero · Accepted solution]

The item I was adding to the cart was a subscription-only variant. As such, it was required to include a sellingPlanId with the line item as below.

  cartCreate(input: {
    lines: [
      {
        merchandiseId: $id, 
        quantity: 1, 
        attributes: [
          {
            key: "AuthenticationCode", 
            value: $auth
          }
        ],
        sellingPlanId: $sellingPlanId
      }
    ], 
    buyerIdentity: {
      email: $email
    }
  }) 

All possible sellingPlans can be found with a product query:

{
  products(first: 4, query: "title:'My subscription Product'") {
    edges {
      node {
        title
        handle
        id
        variants(first: 5) {
          edges {
            node {
              title
              id
              priceV2 {
                amount
              }
              sku
              sellingPlanAllocations(first: 5) {
                edges {
                  node {
                    sellingPlan {
                      id
                      name
                      options {
                        name
                        value
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
    pageInfo {
      hasNextPage
    }
  }
}