do-release-upgrade error when trying to diff modified conf files

I have modified versions of the conf file. I wanted to diff it when prompted by do-release-upgrade. That failed with "sh: 1: pager: not found".

Snippet:
=======================
Setting up smartmontools (7.1-1build1) ...

Configuration file '/etc/default/smartmontools'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** smartmontools (Y/I/N/O/D/Z) [default=N] ? d
sh: 1: pager: not found
diff: standard output: Broken pipe
dpkg: error processing package smartmontools (--configure):
 conffile difference visualizer subprocess returned error exit status 127

ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: smartmontools 7.1-1build1
ProcVersionSignature: Ubuntu 4.15.0-109.110-generic 4.15.18
Uname: Linux 4.15.0-109-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.3
Architecture: amd64
CasperMD5CheckResult: skip
Date: Fri Jul 3 13:32:52 2020
ErrorMessage: conffile difference visualizer subprocess returned error exit status 127
Python3Details: /usr/bin/python3.8, Python 3.8.2, python3-minimal, 3.8.2-0ubuntu2
PythonDetails: /usr/bin/python2.7, Python 2.7.18rc1, python-is-python2, 2.7.17-4
RelatedPackageVersions:
 dpkg 1.19.7ubuntu3
 apt 2.0.2ubuntu0.1
SourcePackage: smartmontools
Title: package smartmontools 7.1-1build1 failed to install/upgrade: conffile difference visualizer subprocess returned error exit status 127
UpgradeStatus: Upgraded to focal on 2020-07-03 (0 days ago)
modified.conffile..etc.default.smartmontools: [modified]
modified.conffile..etc.smartd.conf: [modified]
mtime.conffile..etc.default.smartmontools: 2016-01-25T23:02:07
mtime.conffile..etc.smartd.conf: 2014-10-14T07:00:31