The proposal in JSR 376 is that the identifiers in module names (in source form) must end with end a "Java letter".

As things currently stand in JDK 9 then it's a warning (not an error) at both compile-time and run-time when attempting to compile or resolve a module with a name that doesn't end with a letter.

This issue tracks whatever updates is needed in the implementation to align with the eventual proposal or ruling on this issue.