From Eric Lippert
Now, since this is an error case, surely we can simply fix the bug. We'd be turning a case which is presently an error into something which works. Turning broken code into working code is by definition not a breaking change -- a breaking change is turning working code into broken code, or working code with different semantics.
Someone should tell that to whoever closed this (still lurking) bug: TypeConverter for value types (such as int, bool, short, etc.) return true to IsValid, even if the value is not valid