From d60489f717649db75549c9eb6d4c9c6312c03928 Mon Sep 17 00:00:00 2001
From: "Davide P. Cervone" <dpvc@union.edu>
Date: Tue, 11 Jul 2017 14:30:58 -0400
Subject: [PATCH] Fix problem reported by Alex at
 https://github.com/openwebwork/pg/commit/4973307a4e7c50b6e4f1afdc6f2c44433f7e6f8f#commitcomment-23035223.

---
 macros/PGML.pl | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/macros/PGML.pl b/macros/PGML.pl
index 87ddb36d14..d773fcdbd1 100644
--- a/macros/PGML.pl
+++ b/macros/PGML.pl
@@ -732,8 +732,8 @@ sub combineTopItems {
   my $id = $top->{combine}{$prev->{type}}; my $value; my $inside = 0;
   if ($id) {
     if (ref($id) eq 'HASH') {($id,$value) = %$id; $inside = 1} else {$value = $prev->{$id}}
-    my $topList = substr(($top->topItem || {})->{token} || '',0,2);
-    my $prevList = substr(($prev->topItem || {})->{token} || '',0,2);
+    my $topList = (Value::isa($top,'PGML::Block') ? substr(($top->topItem || {})->{token} || '',0,2) : '');
+    my $prevList = (Value::isa($prev,'PGML::Block') ? substr(($prev->topItem || {})->{token} || '',0,2) : '');
     if (
         $top->{$id} eq $value ||
         ($top->{type} eq 'list' && $top->{bullet} eq 'roman' &&