Unable to retrieve field value from Doc Values in nested function_score query

Test in Elasticsearch CONSOLE

# Elasticsearch version: 5.1.1

DELETE /test-store

PUT /test-store

PUT /test-store/transaction/_mapping
{
  "transaction": {
    "properties": {
      "customer_id": {
        "type": "text"
      },
      "reward_factor": {
        "type": "float"
      },
      "product": {
        "type": "nested",
        "properties": {
          "category": {
            "type": "text"
          },
          "name": {
            "type": "text"
          },
          "quantity": {
            "type": "long"
          },
          "price": {
            "type": "float"
          }
        }
      }
    }
  }
}

PUT /test-store/transaction/1/_create
{
  "customer_id": "customer-1",
  "reward_factor": 1.1,
  "product": [
    {
      "name": "product-1",
      "category": "tech",
      "price": 102.5,
      "quantity": 3
    },
    {
      "name": "product-2",
      "category": "hardware",
      "price": 550.82,
      "quantity": 2
    }
  ]
}

PUT /test-store/transaction/2/_create
{
  "customer_id": "customer-2",
  "reward_factor": 1.05,
  "product": [
    {
      "name": "product-1",
      "category": "tech",
      "price": 72.5,
      "quantity": 2
    },
    {
      "name": "product-2",
      "category": "hardware",
      "price": 300.2,
      "quantity": 1
    }
  ]
}

# Query using function_score to sort transactions by 'reward_factor' adjusted purchase total.
# Result shows all transactions have a _score of '0', further investigation indicates
# "doc['reward_factor'].value" is always '0' which is incorrect
GET /test-store/transaction/_search
{
  "query": {
    "nested": {
      "path": "product",
      "score_mode": "sum",
      "query": {
        "function_score": {
          "query": {
            "match_all": {}
          },
          "boost_mode": "replace",
          "script_score": {
            "script": "doc['product.price'].value * doc['product.quantity'].value * doc['reward_factor'].value"
          }
        }
      }
    }
  }
}