longest path and http_handler prefix option

I forever struggle with prefix option 

from the docs:
Call Pred on any location that is a specialisation of Path. If multiple handlers match, the one with the longest path is used. Options defined with a prefix handler are the default options for paths that start with this prefix


Yet I've defined these handlers

:- http_handler(root(.),
root_handler,
[prefix, priority(-10)]).


:- http_handler(root(map), map_handler, []).

And yet /map is being routed to root_handler

is not /map longer than /  ??
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