We’re one of the many registered vendors with fewer than 4 non-space characters in our ID - 'SIL '. The specs and tests seem to prefer a fully padded four-character string, not ‘SIL’ (without the trailing space. However I can’t seem to add the space by editing the vendorID custom parameter. It always loses the trailing space.
All values are stored without trailing spaces. In most other places they are not allowed. But you don’t need to worry. The value is padded with spaces on export.