Hi, Dupond.
Rusty's answer is precise.
Just wanted to outline, as Rusty said, that you will find it incorrectly used by a lot of native Spanish speakers. Both written and spoken. Either including "de" to indicate obligation or omitting "de" to express probability or supposition.
So, stay firm, and use it correctly
, no matter who you hear using it the other way around. It is not uncommon to hear well educated native speakers making this mistake.
Un abrazo,