Screwing Robe Hook to Solid Bathroom Door

[paulb01]

Bought a simple hook from B&Q - one of these:

[url]http://www.diy.com/departments/bq-zinc-alloy-hat-coat-hook/240804_BQ.prd[/url]

to attach to the back of my bathroom door. Comes with wall plugs. Note in the package says "If fitting hook / hook rail to a door or non-solid wall, additional fixing hardware (not included) will be required."

Doesn't differentiate for solid/hollow-core (mine are solid), so any idea what additional fixing hardware it refers to? Or would you just ignore and screw directly into the door? Thanks in advance.