diff --git a/include/functions_entries.inc.php b/include/functions_entries.inc.php
index a5357604..ed6ff026 100644
--- a/include/functions_entries.inc.php
+++ b/include/functions_entries.inc.php
@@ -834,7 +834,7 @@ function &serendipity_searchEntries($term, $limit = '', $searchresults = '') {
     $limit = serendipity_db_limit_sql($limit);
 
     $term = serendipity_db_escape_string($term);
-    $cond = array();
+    $cond = array('having' => '');
     $relevance_enabled = false;
     if ($serendipity['dbType'] == 'postgres' ||
         $serendipity['dbType'] == 'pdo-postgres') {
diff --git a/include/genpage.inc.php b/include/genpage.inc.php
index 147f0c7b..398f440d 100644
--- a/include/genpage.inc.php
+++ b/include/genpage.inc.php
@@ -91,7 +91,7 @@ switch ($serendipity['GET']['action']) {
             array(
                 'content_message'      => sprintf(YOUR_SEARCH_RETURNED_BLAHBLAH, '<span class="searchterm">' . $serendipity['GET']['searchTerm'] . '</span>', '<span class="searchresults">' . serendipity_getTotalEntries() . '</span>'),
                 'searchresult_results' => true,
-                'searchresult_fullentry' => $serendipity['GET']['fullentry']
+                'searchresult_fullentry' => $serendipity['GET']['fullentry'] ?? null
             )
         );
 
diff --git a/templates/bootstrap4/commentform.tpl b/templates/bootstrap4/commentform.tpl
index 51ee0e34..a4023724 100644
--- a/templates/bootstrap4/commentform.tpl
+++ b/templates/bootstrap4/commentform.tpl
@@ -8,18 +8,18 @@
         <input type="hidden" name="serendipity[entry_id]" value="{$commentform_id}">
 
         <div class="form-group">
-            <label for="serendipity_commentform_name">{$CONST.NAME}{if $required_fields.name}*{/if}</label>
-            <input id="serendipity_commentform_name" class="form-control" type="text" name="serendipity[name]" value="{$commentform_name}"{if $required_fields.name} required{/if}>
+            <label for="serendipity_commentform_name">{$CONST.NAME}{if isset($required_fields.name) and $required_fields.name}*{/if}</label>
+            <input id="serendipity_commentform_name" class="form-control" type="text" name="serendipity[name]" value="{$commentform_name}"{if isset($required_fields.name) and $required_fields.name} required{/if}>
         </div>
 
         <div class="form-group">
-            <label for="serendipity_commentform_email">{$CONST.EMAIL}{if $required_fields.email}*{/if}</label>
-            <input id="serendipity_commentform_email" class="form-control" type="email" name="serendipity[email]" value="{$commentform_email}"{if $required_fields.email} required{/if}>
+            <label for="serendipity_commentform_email">{$CONST.EMAIL}{if isset($required_fields.email) and $required_fields.email}*{/if}</label>
+            <input id="serendipity_commentform_email" class="form-control" type="email" name="serendipity[email]" value="{$commentform_email}"{if isset($required_fields.email) and $required_fields.email} required{/if}>
         </div>
 
         <div class="form-group">
-            <label for="serendipity_commentform_url">{$CONST.HOMEPAGE}{if $required_fields.url}*{/if}</label>
-            <input id="serendipity_commentform_url" class="form-control" type="url" name="serendipity[url]" value="{$commentform_url}"{if $required_fields.url} required{/if}>
+            <label for="serendipity_commentform_url">{$CONST.HOMEPAGE}{if isset($required_fields.url) and $required_fields.url}*{/if}</label>
+            <input id="serendipity_commentform_url" class="form-control" type="url" name="serendipity[url]" value="{$commentform_url}"{if isset($required_fields.url) and $required_fields.url} required{/if}>
         </div>
 
         <div class="form-group">
@@ -28,8 +28,8 @@
         </div>
 
         <div class="form-group">
-            <label for="serendipity_commentform_comment">{$CONST.COMMENT}{if $required_fields.comment}*{/if}</label>
-            <textarea id="serendipity_commentform_comment" class="form-control" rows="10" name="serendipity[comment]"{if $required_fields.comment} required{/if}>{$commentform_data}</textarea>
+            <label for="serendipity_commentform_comment">{$CONST.COMMENT}{if isset($required_fields.comment) and $required_fields.comment}*{/if}</label>
+            <textarea id="serendipity_commentform_comment" class="form-control" rows="10" name="serendipity[comment]"{if isset($required_fields.comment) and $required_fields.comment} required{/if}>{$commentform_data}</textarea>
         </div>
         {serendipity_hookPlugin hook="frontend_comment" data=$commentform_entry}
     {if $is_commentform_showToolbar}
diff --git a/templates/bootstrap4/comments.tpl b/templates/bootstrap4/comments.tpl
index f94a357c..d4fe87b3 100644
--- a/templates/bootstrap4/comments.tpl
+++ b/templates/bootstrap4/comments.tpl
@@ -13,7 +13,7 @@
         {/if}
         </div>
     {if $entry.allow_comments AND $comment.body != 'COMMENT_DELETED'}
-        <a id="serendipity_reply_{$comment.id}" class="comment_reply btn btn-outline-primary btn-sm" href="#serendipity_CommentForm" onclick="document.getElementById('serendipity_replyTo').value='{$comment.id}'; {$comment_onchange}">{$CONST.REPLY}</a>
+        <a id="serendipity_reply_{$comment.id}" class="comment_reply btn btn-outline-primary btn-sm" href="#serendipity_CommentForm" onclick="document.getElementById('serendipity_replyTo').value='{$comment.id}'; {if isset($comment_onchange)}{$comment_onchange}{/if}">{$CONST.REPLY}</a>
         <div id="serendipity_replyform_{$comment.id}"></div>
     {/if}
     </li>
diff --git a/templates/bootstrap4/config.inc.php b/templates/bootstrap4/config.inc.php
index 3d1d9252..62fc27f6 100644
--- a/templates/bootstrap4/config.inc.php
+++ b/templates/bootstrap4/config.inc.php
@@ -103,9 +103,9 @@ $template_config = array(
 );
 
 $template_global_config = array('navigation' => true);
-$template_loaded_config = serendipity_loadThemeOptions($template_config, $serendipity['smarty_vars']['template_option'], true);
+$template_loaded_config = serendipity_loadThemeOptions($template_config, $serendipity['smarty_vars']['template_option'] ?? '', true);
 serendipity_loadGlobalThemeOptions($template_config, $template_loaded_config, $template_global_config);
 
-if ($_SESSION['serendipityUseTemplate']) {
+if (isset($_SESSION['serendipityUseTemplate']) && $_SESSION['serendipityUseTemplate']) {
     $template_loaded_config['use_corenav'] = false;
 }
diff --git a/templates/bootstrap4/entries.tpl b/templates/bootstrap4/entries.tpl
index 97cc85d0..6d498a26 100644
--- a/templates/bootstrap4/entries.tpl
+++ b/templates/bootstrap4/entries.tpl
@@ -34,7 +34,7 @@
             {if $entry.has_comments}
                 <li class="post_comments d-inline-block"><svg class="icon-comments" role="img" viewbox="0 0 1792 1792" width="1792" height="1792" aria-labelledby="title"><title id="title">{$entry.label_comments}</title><use xlink:href="{$serendipityHTTPPath}templates/{$template}/img/icons.svg#comments"></use></svg><a href="{$entry.link}#comments" title="{$entry.comments} {$entry.label_comments}{if $entry.has_trackbacks}, {$entry.trackbacks} {$entry.label_trackbacks}{/if}">{$entry.comments}</a></li>
             {/if}
-            {if $entry.freetag.tags.tags}
+            {if isset($entry.freetag.tags.tags) && $entry.freetag.tags.tags}
                 <li class="post_tags d-inline-block"><svg class="icon-tag" role="img" viewbox="0 0 1792 1792" width="1792" height="1792" aria-labelledby="title"><title id="title">Tags</title><use xlink:href="{$serendipityHTTPPath}templates/{$template}/img/icons.svg#tag"></use></svg>{foreach from=$entry.freetag.tags.tags item="tag"}{$tag} {/foreach}</li>
             {/if}
             </ul>
@@ -109,8 +109,8 @@
     <p class="alert alert-info" role="alert">{$CONST.NO_ENTRIES_TO_PRINT}</p>
     {/if}
 {/foreach}
-{if not $is_preview}
-    {if $staticpage_pagetitle == ''}
+{if not $is_preview and ! $is_single_entry}
+    {if ! isset($staticpage_pagetitle) || $staticpage_pagetitle == ''}
     <nav aria-label="{$footer_info}" title="{$footer_info}">
         <ul class="pagination justify-content-between">
             <li class="page-item{if !$footer_prev_page} disabled{/if}">
diff --git a/templates/bootstrap4/index.tpl b/templates/bootstrap4/index.tpl
index 28bc8dc9..8a5de8f5 100644
--- a/templates/bootstrap4/index.tpl
+++ b/templates/bootstrap4/index.tpl
@@ -6,7 +6,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>{$head_title|default:$blogTitle}{if $head_subtitle} | {$head_subtitle}{/if}</title>
     <meta name="generator" content="Serendipity v.{$head_version}">
-{if ($view == "entry" || $view == "start" || $view == "feed" || $view == "plugin" || $staticpage_pagetitle != "" || $robots_index == 'index')}
+{if ($view == "entry" || $view == "start" || $view == "feed" || $view == "plugin" || (isset($staticpage_pagetitle) && $staticpage_pagetitle != "") || (isset($robots_index) and $robots_index == 'index'))}
     <meta name="robots" content="index,follow">
 {else}
     <meta name="robots" content="noindex,follow">