Server error in backwards search

curl -X GET "https://procedure-staging.prozorro.sale/api/search/byLastModified/2019?backward=true" -H "accept: application/json"

[2019-11-21T12:16:53 +0000] | aiohttp.server | ERROR - Error handling request
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/aiohttp/web_protocol.py", line 418, in start
    resp = await task
  File "/usr/local/lib/python3.8/site-packages/aiohttp/web_app.py", line 458, in _handle
    resp = await handler(request)
  File "/search/prozorro_sale/search/api.py", line 72, in search_by_last_modified
    backward = int(request.query.get('backward', '0'))
ValueError: invalid literal for int() with base 10: 'true'
[2019-11-21T12:16:53 +0000] | aiohttp.access | INFO - "192.168.94.34 GET /api/search/byLastModified/2019?backward=true HTTP/1.1 500 244 https://procedure-staging.prozorro.sale/api/doc X-ResultLength: None Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.97 Safari/537.36 0.001270"