Namespace names (id) can contain only alphanumeric characters or '_'.
This information is not surfaced to the user, until the user clicks [Finish] and an error is displayed.
Hint text in the "name" text field is changed to display:
"Use only alphanumeric characters or '_' "
When the user selects either [Finish] or [Next>] the system checks the name and surface an error if the name doesn't meet the requirements.