unable to install lldb-9 from apt.llvm.org

I’m having problems installing the current (release) for lldb-9 from apt.llvm.org (for ubuntu 18.04/bionic):

apt-get -y install lldb-9 python-lldb-9 liblldb-9-dev

results in:

The following packages have unmet dependencies:
lldb-9 : Depends: llvm-9-dev but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Oddly, llvm-9-dev DOES seem to be installed:

Setting up llvm-9-dev (1:9~svn372167-1~exp1~20190917193124.56) …

This was not happening with the pre-release packages for 9. Any ideas what’s changed?

Mike Ellery