> When in Scala code null is passed it is a bug, thus I wouldn't react on that. On the other side, I have no idea if this method gets called from Java which could pass null.

We are in Eclipse land, where ``null`` has often a semantic meaning, i.e., no value. However, as a general rule, I agree that if ``null`` is a valid value, we should say so in a ScalaDoc comment. Anyway, in the specific case, I think it's ok to change the contract of this method because I don't think it should ever accept ``null`` regions. The idea of throwing an exception on ``null`` values was simply to inform callers of the semantic change and provide a more meaningful message than just a NPE.

> Btw, instead of throwing an exception, one can just use require(cond, "error"). ;)

[That results in an exception being thrown](https://github.com/scala/scala/blob/v2.10.2/src/library/scala/Predef.scala#L221), and it's indeed an ``IllegalArgumentException``, so we agree on this.