Filings Pagination Issue

When I make a call to the filings history endpoint with a starting index of 0 the returned start_index is of type Integer.

https://api.company-information.service.gov.uk/company/07118675/filing-history?items_per_page=20&start_index=0

{
    "total_count": 81,
    "filing_history_status": "filing-history-available",
    "items_per_page": 20,
    "start_index": 0,
    "items": [
....
    ]
}

However when I fetch the next 20 results the start_index then changes to a string.
{
“start_index”: “20”,
“items”: [

],
“total_count”: 81,
“filing_history_status”: “filing-history-available”,
“items_per_page”: 20
}