A few months ago, we noted that a Yelp employee’s online “negative review” of her employer might be protected activity under the National Labor Relations Act (NLRA), given that the National Relations Labor Board (NLRB) has become increasingly aggressive in protecting an employee’s right to discuss working conditions in a public forum, even when that discussion involves obscenities or disparaging the employer. This trend has prompted us to report previously on the death of courtesy and civility under the NLRA.

Now the NLRB has confirmed that it is not only courtesy and civility that have passed away—a “positive work environment” has perished with them.

A recent NLRB decision found that T-Mobile’s employee handbook violated the NLRA by requiring employees “to maintain a positive work environment by communicating in a manner that is conducive to effective working relationships with internal and external customers, clients, co-workers, and management.”

According to the NLRB, employees could reasonably construe such a rule “to restrict potentially controversial or contentious communications,” including communications about labor disputes and working conditions that are protected under the NLRA. The NLRB concluded that employees rightly feared their employer would consider such communications to be inconsistent with a “positive work environment.” Similarly, the NLRB struck down T-Mobile’s rules against employees “arguing” and making “detrimental” comments about the company.

The main sticking point appears to be requiring employees to be “positive” towards co-workers and management. Earlier NLRB cases have indicated that requiring employees to be courteous only towards customers may not set off as many NLRB alarm bells. Nonetheless, employers should tread carefully—and try not to be too cheerful. Encouraging a positive attitude among employees could have negative results.