bash uninitialized memory access

The following gives an error message that suggests that bash is reading from uninitialized memory:

bash -c 'foo() { declare -n bar=baz; declare -A bar; bar[x]=y; }; foo'
environment: line 0: `�d�K�U': not a valid identifier

The short string of garbled characters is different on each run.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: bash 4.4.18-2ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-47.50-generic 4.15.18
Uname: Linux 4.15.0-47-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri May 3 11:07:11 2019
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: bash
UpgradeStatus: Upgraded to bionic on 2019-04-02 (30 days ago)