foo is not a valid TAG

tag: URIs must conform to the TAG URI specification.

Ensure that the URI:

1. Starts with tag
2. Followed by a :
3. Followed by an authority name of either a DNS name or an email address
4. Followed by a ,
5. Followed by the date at which this tag scheme was first used. Note that this is the date at which the tag scheme was minted, not the date of the specific entry or item. Format is YYYY-MM-DD with the month and day parts being optional.
6. Followed by a a :
7. Followed by any number of characters which are valid in a URI. Special characters may be escaped using percent encoding.