Thx to Frances, Max, Stu & Greg ....
@MVerta .... yep, ....mental typo. hehe
Will ..... that could either be something big and dark in the hdri or as you stated ---- an area where the HDRi isn't mapping. In this case, ...I'm not sure -- but I'll check.
I'm not sure why your having issues though ..... if you drop in a longitudinal map, it should default map to spherical .... ??
maybe post an image ... ?