Trouble adding a new label to [[Module:category tree/topic cat/data/Place names]]

Hi CodeCat. I tried adding "provinces of the Roman Empire" as a label to Module:category tree/topic cat/data/Place names, but {{topic cat|…|provinces of the Roman Empire}} ended up generating the error message "Lua error in Module:category_tree at line 126: attempt to index field 'name' (a nil value)" in Category:en:Provinces of the Roman Empire, Category:fr:Provinces of the Roman Empire, and Category:la:Provinces of the Roman Empire. I don't understand what I've done wrong; could you explain to me how to fix this, please?

 — I.S.M.E.T.A.14:09, 3 March 2015

It's because there is no category "Roman Empire" yet.

CodeCat17:09, 3 March 2015

Thanks. I've fixed it now. :-)

 — I.S.M.E.T.A.19:45, 3 March 2015