Getting "argument cannot be null" error message wh

I am using below progress code for calling REST API and getting error message as "argument cannot be null". Progress version is 11.6.3.

oRequest =
RequestBuilder
:POST(chhttpUrl,oRequestBody)
:ContentType('application/x-www-form-urlencoded')
:AcceptJson()
:UsingCredentials(oCredentials)
:REQUEST.

oResponse = oClient:EXECUTE(oRequest).

Can you please provide the deep detail, how can I resolve this issue?

Thanks in advance.