SVN precommit and nullable return types

When committing my plugin to the repo, I'm getting an error that looks like this:

Commit blocked by pre-commit hook (exit code 1) with output:


***********************************
PHP error in: path/to/file.php:


Parse error: syntax error, unexpected '?' in path/to/file.php on line 36
Errors parsing path/to/filephp

Examining the line in question, I see that the ? is part of one of PHP 7.1's nullable return types:

static function foo($bar=null): ?string { ... }

Note that my readme.txt file correctly notes the minimum PHP version, if that makes any difference to the parser. If not, do I need to remove all my nullable return types for now?