`Moltin.Products.CreateRelationships(productId, 'product-variation', variationIds)` returns `{ data: null }`

Problem

I'm trying to link product variation with product:

const productId = '797cd13e-edbb-4ee1-a47e-2d8ff9048ab7'
const variationIds = [ '44de8b4f-34ca-457c-99af-9832788c1af0' ]
const moltin = gateway({
  client_id: options.clientId,
  client_secret: options.clientSecret,
})

console.log('LLL linkProductWithVariation', productId, variationIds)

moltin.Products.CreateRelationships(productId, 'product-variation', variationIds)
  .then(response => {
    console.log('LLL linkProductWithVariation response:', response)
    return response
  })
  .catch(error => {
    console.log('LLL linkProductWithVariation ERROR', error)
    throw new Error(error)
  })

In the terminal i get:

LLL linkProductWithVariation 797cd13e-edbb-4ee1-a47e-2d8ff9048ab7 [ '44de8b4f-34ca-457c-99af-9832788c1af0' ]
LLL linkProductWithVariation response: { data: null }

Method returned { data: null } and Product didn't linked with provided Variation, which leads to the following Build Child Products method 409 error.

Expected behavior

Method should return product object, like it do when I use REST api: REST_api_response After it Product should link with provided Variation and the following Build Child Products method should build child products.

Extra

Here is full response object from provided screenshot:

{
"type": "product",
"id": "797cd13e-edbb-4ee1-a47e-2d8ff9048ab7",
"name": "new_product_3",
"slug": "new_product_3",
"sku": "new_product_3",
"manage_stock": true,
"description": "new_product_3",
"price": [
    {
        "amount": 100000,
        "currency": "RUB",
        "includes_tax": true
    }
],
"status": "draft",
"commodity_type": "physical",
"meta": {
    "timestamps": {
        "created_at": "2019-07-24T16:24:11+00:00",
        "updated_at": "2019-07-24T16:24:11+00:00"
    },
    "display_price": {
        "with_tax": {
            "amount": 100000,
            "currency": "RUB",
            "formatted": "1000,00 ₽"
        },
        "without_tax": {
            "amount": 100000,
            "currency": "RUB",
            "formatted": "1000,00 ₽"
        }
    },
    "stock": {
        "level": 123,
        "availability": "in-stock"
    },
    "variations": [
        {
            "id": "44de8b4f-34ca-457c-99af-9832788c1af0",
            "name": "defaultSizeRange",
            "options": [
                {
                    "id": "eee6d36b-0671-410b-97f0-327d6c56766b",
                    "name": "S",
                    "description": "Small"
                },
                {
                    "id": "8003a997-0432-4299-958d-9d0bb799d765",
                    "name": "M",
                    "description": "Medium"
                },
                {
                    "id": "a0b55889-7a1b-4220-8a2a-5fcdca004af7",
                    "name": "L",
                    "description": "Large"
                }
            ]
        }
    ]
},
"relationships": {
    "variations": {
        "data": [
            {
                "type": "product-variation",
                "id": "44de8b4f-34ca-457c-99af-9832788c1af0"
            }
        ]
    }
}
}

I checked the source code looks like it's okay, maybe there's a something wrong on the backend?
Question

Is there some problem with API or did I get something wrong? How can I fix it?

moltin / js-sdk