how to fetch entire big ip configuration file

I tried changing content type to application/json and added content range header but it returns "string index out of range". Here is the request/response pair (below)

Request

GET https://172.18.20.98/mgmt/shared/file-transfer/ucs-downloads/bigipconfig.ucs HTTP/1.1

Content-Type: application/json

X-F5-Auth-Token: BS3ZK3KEYLVEXBH2VD4R4RAUGD

Content-Range: 512

Host: 172.18.20.98

Response

HTTP/1.1 400 Bad Request

Date: 12 Jul 2019 16:32:16 UTC

Server: com.f5.rest.common.RestRequestSender

X-Frame-Options: SAMEORIGIN

Strict-Transport-Security: max-age=16070400; includeSubDomains

Pragma: no-cache

Cache-Control: no-store, no-cache, must-revalidate

Expires: -1

Content-Length: 132

Content-Type: application/json

Content-Range: 512

Local-Ip-From-Httpd: 172.18.20.98

X-Forwarded-Server: localhost.localdomain

X-Forwarded-Proto: http

X-Forwarded-Host: 172.18.20.98

X-Content-Type-Options: nosniff

X-XSS-Protection: 1; mode=block

Content-Security-Policy: default-src 'self' 'unsafe-inline' 'unsafe-eval'; img-src 'self' http://127.4.1.1 http://127.4.2.1

Connection: close

{"code":400,"message":"String index out of range: -1","referer":"172.18.20.97","restOperationId":136890,"kind":":resterrorresponse"}