Express "Class argument may not be null" ?

[Johan Engelen via Digitalmars-d-learn]

Hi all,
  How would you express the function interface intent that a 
reference to a class may not be null?
For a function "void foo(Klass)", calling "foo(null)" is valid. 
How do I express that that is invalid? (let's leave erroring with 
a compile error aside for now)

Something equivalent to C++'s pass by reference: "void 
foo(Klass&)".

(note: I mean D classes, for structs "ref" works)

Thanks,
  Johan