Error when running ab #14

In Saturation, Instruments to Measure Golden Signals of the hands-on lab 8, when running the following command to apply the load:
ab -n 50000 -c 100 http:‌//localhost:5000/users

This leads to an error:
apr_socket_connect(): Invalid argument (22)

I am on OSX, and it seems the host name is resolved as a local IPv6 address.

This error can be avoided by:
ab -n 50000 -c 100 http://127.0.0.1:5000/users

Reference: https://serverfault.com/questions/236623/receiving-error-apr-socket-connect-invalid-argument-22-when-running-apache