Error 400 while integrating Go-Swagger based API with Zapier

I have created & deployed a REST API server using Go, gin-gonic, swagger documentation.

I have also created some endpoints for sending GET & POST requests to that server.

For authentication, I have set a custom authentication header key "Authorization" with its value input from the user as an API key (Through this API key, my REST API server sends further request to a third-party cloud storage service)

I am able to authenticate & send GET/POST requests successfully from browser (Swagger API documentation webpage), Postman client and it works absolutely fine.

Now I was trying to integrate that server's API to Zapier.

But when I test my authentication by entering the API key, it displays following error with some node.js event issues:

We hit an error adding your new account
authentication failed: The app returned "400". What happened (You are seeing this because you are an admin): 
Stack trace: 
ResponseError: 
{
       "status":400,
       "headers":
       {
               "content-type":null
       },
       "content":"",
       "request":
       {
               "url":"http://<my API endpoint URL>/transfer/foldername/filename.jpg"
       }
} 
at Object.throwForStatus (/var/task/node_modules/zapier-platform-core/src/http-middlewares/after/throw-for-status.js:7:11) 

From previous event:
at executeHttpRequest (/var/task/node_modules/zapier-platform-core/src/execute.js:22:6)
at execute (/var/task/node_modules/zapier-platform-core/src/execute.js:90:12)
at input (/var/task/node_modules/zapier-platform-core/src/create-command-handler.js:29:14) 
at Object.beforeMiddleware.then.newInput (/var/task/node_modules/zapier-platform-core/src/middleware.js:90:22) 
at bound (domain.js:402:14)
at Object.runBound (domain.js:415:12) 

From previous event: 
at input (/var/task/node_modules/zapier-platform-core/src/middleware.js:89:45) 
at loadApp.then.appRaw (/var/task/node_modules/zapier-platform-core/src/tools/create-lambda-handler.js:215:18) 
at bound (domain.js:402:14) at runBound (domain.js:415:12) 
at runCallback (timers.js:705:18) 
at tryOnImmediate (timers.js:676:5) 
at processImmediate (timers.js:658:5) 
at process.topLevelDomainCallback (domain.js:126:23) 

From previous event: 
at Domain.handlerDomain.run (/var/task/node_modules/zapier-platform-core/src/tools/create-lambda-handler.js:200:10) 
at Domain.run (domain.js:349:14) 
at Runtime.handler (/var/task/node_modules/zapier-platform-core/src/tools/create-lambda-handler.js:192:19) 
at Runtime.handleOnce (/var/runtime/Runtime.js:66:25)

Am I missing something here?