joined tables with USING and GROUPBY on the USING() column

I'll start by saying that I'm sure I'm missing something obvious...

I have a query that is working fine on all my servers except one. The
only obvious difference is that the failing one is running 9.5.3,
while most are running 9.5.4, but since the query works on a 9.5.1 box
I also have I can't imagine that's the cause.

I can simplify the query (and still retain the error) down to:

SELECT results.token FROM results INNER JOIN tokens USING(token) GROUP BY token;

ERROR: column "results.token" must appear in the GROUP BY clause or be
used in an aggregate function

All well and good, and I'm happy enough to change the query's GROUP BY
to include the table name, but it's confusing me how it works OK on
all servers except one.

Is there some configuration option I'm missing?

Thanks!

Geoff