CommandException: No URLs matched when passing URLs to rm from stdin

I'm trying to use `gsutil rm -I` and pass a list of URLs to delete through stdin.

For an existing directory in, say, `gs://test-bucket/test-dir`, these are some commands I've tried:
```
# verify directory exists
$ gsutil ls -d gs://test-bucket/test-dir
gs://test-bucket/test-dir/

$ echo "gs://test-bucket/test-dir" | gsutil -m rm -r -I
CommandException: No URLs matched

$ echo gs://test-bucket/test-dir | gsutil -m rm -r -I
CommandException: No URLs matched

$ gsutil -m rm -r -I <<< "gs://test-bucket/test-dir"
CommandException: No URLs matched

$ gsutil ls -d gs://test-bucket/test-dir | gsutil -m rm -r -I
CommandException: No URLs matched
```

Am I missing something here?