diff --git a/lib/LaTeXML/Core/Document.pm b/lib/LaTeXML/Core/Document.pm
index aa04f1db4..637945909 100644
--- a/lib/LaTeXML/Core/Document.pm
+++ b/lib/LaTeXML/Core/Document.pm
@@ -498,10 +498,10 @@ sub serialize_aux {
       (map { $_ . '="' . $atnodes{$_} . '"' } sort keys %atnodes)
     );
     my $noindent_children = ($heuristic
-        # This emulates libxml2's heuristic
-        #     ? $noindent || grep { $_->nodeType != XML_ELEMENT_NODE } @children
+      # This emulates libxml2's heuristic
+      #     ? $noindent || grep { $_->nodeType != XML_ELEMENT_NODE } @children
       ? $noindent || grep { $_->nodeType == XML_TEXT_NODE } @children
-        # This is the "Correct" way to determine whether to add indentation
+      # This is the "Correct" way to determine whether to add indentation
       : $model->canContain(getNodeQName($self, $node), '#PCDATA'));
     return join('',
       ($noindent ? '' : $indent), $start,
@@ -1376,12 +1376,15 @@ sub makeError {
 # [xml:id and namespaced attributes are always allowed]
 sub setAttribute {
   my ($self, $node, $key, $value) = @_;
-  return if (ref $value) && ((!blessed($value)) || !$value->can('toAttribute'));
   if (ref $value) {
     if ($key eq '_box') {
       return $self->setNodeBox($node, $value); }
     elsif ($key eq '_font') {
       return $self->setNodeFont($node, $value); }
+    elsif ((!blessed($value)) || !$value->can('toAttribute')) {
+      Warn('unexpected', (ref $value), $self,
+        "Don't know how to encode $value as an attribute value");
+      return; }
     else {
       $value = $value->toAttribute; } }
   if ((defined $value) && ($value ne '')) {    # Skip if `empty'; but 0 is OK!