Why does Enumerable.All return true for an empty sequence?

var strs = new Collection<string>();
bool b = strs.All(str => str == "ABC");

The code creates an empty collection of string, then tries to determine if all the elements in the collection are "ABC". If you run it, b will be true.

But the collection does not even have any elements in it, let alone any elements that equal to "ABC".

Is this a bug, or is there a reasonable explanation?