Gitlab API adding members to a project

I'm having problems when trying to add users to a Gitlab project using the Gitlab API. I'm using Gitlab version 8.11.0 on CentOS 6.8 and I'm not sure if it is a bug or something I am doing wrong. The following works fine and gives me a list of project members:

curl --header "PRIVATE-TOKEN: top_secret" "https://thegitlabserver.com/api/v3/projects/1/members"

but when trying to add members to a project with the following:

curl --request POST --header "PRIVATE-TOKEN: top_secret" "https://thegitlabserver.com/api/v3/projects/1/members/myusername?access_level=30"

I get an error message:

{"error":"405 Not Allowed"}

Using the web interface I am able to add the user to the project. Any help would be greatly appreciated.