tns package-manager fails with error

[rosen-vladimirov]

Environment
Provide version numbers for the following components (information can be retrieved by running tns info in your project folder or by inspecting the package.json of the project):

• CLI: 6.4.0
• Cross-platform modules: Not applicable
• Android Runtime: Not applicable
• iOS Runtime: Not applicable
• Plugin(s): Not applicable

Describe the bug
tns package-manager fails with error:
The input is not valid sub-command for 'package-manager' command.
Run 'tns package-manager --help' for more information.

To Reproduce
tns package-manager

Expected behavior
Print the currently used package-manager.