GDELT set SourceCountry in docs api call

Question

How can I change the SourceCountry in the GDELT Docs API? In the documentation I found the possibility to set the SourceCountry. But when I tried it with this little example, it doesn't work: Without SourceCountry: https://api.gdeltproject.org/api/v2/doc/doc?format=JSON&mode=TimelineTone&query="Jimmy Carter"

With SourceCountry

https://api.gdeltproject.org/api/v2/doc/doc?format=JSON&SourceCountry=france&mode=TimelineTone&query="Jimmy Carter"

So how is it possible to filter on a specific country?