Creating new tag with spaces in name doesn't get assigned

When assigning a new tag (not pre-existing) to a message, I am prompted to confirm that I want to create a new tag:

"No tag named “newtag” currently exists. Do you want to create it?"

If I select "Create Tag", the tag is created, and assigned to the selected message (this works as desired).

However, if the tag name contains spaces, the new tag is not assigned to the selected message.