Issue with mean x, over(y) command in Stata 16

There is an issue with the mean command in Stata 16, which does not exist in Stata 15.

Here is an example:
mean rel_percapgdp, over(war_year)

the variable war_year ranges from -20 to 20.
I get the following error message:

invalid over() option;
war_year: factor variables may not contain negative values
r(452);

The same command in Stata 15 provides valid results.

Any suggestions?
Zeev