Plugin doesn't populate credentials drop down

When configuring GitLab Servers in the Jenkins global configuration, the credentials dropdown doesn't populate with system credentials.

We have a private GitLab server on the same network as our Cloudbees instance. After entering the GitLab URL in the URL field, the UI shows an "Invalid GitLab Server URL".

I did some digging into the source code and figured out that it tries to hit the /projects API of the gitlab instance in order to validate the location. For our instance, you can't hit the /projects API unless you're authorized. Since the credentials drop down seemingly doesn't populate unless you've entered a "valid" gitlab URL (or maybe not at all?), we can't select creds to authorize the GitLab validity check with.

Also, adding new credentials from this configuration doesn't populate the drop down, but it does at them to the list of system credentials for the Jenkins managed master.

Screen shot below: