Error msg: Unable to verify the first certificate with https request

Hi guys

Package.json
"jest": "24.6.0",
"supertest": "^4.0.2"

it('should return success on valid request', function(done) {
supertest('https://xxxxxx')
.get('xxxxxx')
.set('Accept', 'application/json')
.expect(200)
.end(done);
});

error msg
unable to verify the first certificate

Have tried process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0'; before test as well. This solution also didnt solve the issue.

Curl req working fine.

kindly advice