diff --git a/lib/navigationlib.php b/lib/navigationlib.php
index 13711e0bcc1..eeb404c296d 100644
--- a/lib/navigationlib.php
+++ b/lib/navigationlib.php
@@ -544,7 +544,11 @@ class navigation_node implements renderable {
         if (in_array($class, $this->classes)) {
             $key = array_search($class,$this->classes);
             if ($key!==false) {
+                // Remove the class' array element.
                 unset($this->classes[$key]);
+                // Reindex the array to avoid failures when the classes array is iterated later in mustache templates.
+                $this->classes = array_values($this->classes);
+
                 return true;
             }
         }
diff --git a/theme/boost/templates/flat_navigation.mustache b/theme/boost/templates/flat_navigation.mustache
index a5913267597..31bfacaab4e 100644
--- a/theme/boost/templates/flat_navigation.mustache
+++ b/theme/boost/templates/flat_navigation.mustache
@@ -62,7 +62,7 @@
 <nav class="list-group mt-1" aria-label="{{get_collectionlabel}}">
     {{/showdivider}}
     {{#action}}
-    <a class="list-group-item list-group-item-action {{#isactive}}active{{/isactive}}" href="{{{action}}}" data-key="{{key}}" data-isexpandable="{{isexpandable}}" data-indent="{{get_indent}}" data-showdivider="{{showdivider}}" data-type="{{type}}" data-nodetype="{{nodetype}}" data-collapse="{{collapse}}" data-forceopen="{{forceopen}}" data-isactive="{{isactive}}" data-hidden="{{hidden}}" data-preceedwithhr="{{preceedwithhr}}" {{#parent.key}}data-parent-key="{{.}}"{{/parent.key}}>
+    <a class="list-group-item list-group-item-action {{#isactive}}active{{/isactive}} {{#classes}}{{.}} {{/classes}}" href="{{{action}}}" data-key="{{key}}" data-isexpandable="{{isexpandable}}" data-indent="{{get_indent}}" data-showdivider="{{showdivider}}" data-type="{{type}}" data-nodetype="{{nodetype}}" data-collapse="{{collapse}}" data-forceopen="{{forceopen}}" data-isactive="{{isactive}}" data-hidden="{{hidden}}" data-preceedwithhr="{{preceedwithhr}}" {{#parent.key}}data-parent-key="{{.}}"{{/parent.key}}>
         <div class="ml-{{get_indent}}">
             <div class="media">
             {{#icon.pix}}
@@ -76,7 +76,7 @@
     </a>
     {{/action}}
     {{^action}}
-    <div class="list-group-item" data-key="{{key}}" data-isexpandable="{{isexpandable}}" data-indent="{{get_indent}}" data-showdivider="{{showdivider}}" data-type="{{type}}" data-nodetype="{{nodetype}}" data-collapse="{{collapse}}" data-forceopen="{{forceopen}}" data-isactive="{{isactive}}" data-hidden="{{hidden}}" data-preceedwithhr="{{preceedwithhr}}" {{#parent.key}}data-parent-key="{{.}}"{{/parent.key}}>
+    <div class="list-group-item {{#classes}}{{.}} {{/classes}}" data-key="{{key}}" data-isexpandable="{{isexpandable}}" data-indent="{{get_indent}}" data-showdivider="{{showdivider}}" data-type="{{type}}" data-nodetype="{{nodetype}}" data-collapse="{{collapse}}" data-forceopen="{{forceopen}}" data-isactive="{{isactive}}" data-hidden="{{hidden}}" data-preceedwithhr="{{preceedwithhr}}" {{#parent.key}}data-parent-key="{{.}}"{{/parent.key}}>
         <div class="ml-{{get_indent}}">
             <div class="media">
             {{#icon.pix}}