diff --git a/lang/en/search.php b/lang/en/search.php
index 096722d5a91d8..ebd0f5a32e7b4 100644
--- a/lang/en/search.php
+++ b/lang/en/search.php
@@ -51,6 +51,7 @@
 $string['enginenotselected'] = 'You have not selected any search engine.';
 $string['engineserverstatus'] = 'The search engine is not available. Please contact your administrator.';
 $string['enteryoursearchquery'] = 'Enter your search query';
+$string['error_indexing'] = 'An error occurred while indexing';
 $string['errors'] = 'Errors';
 $string['errorareanotavailable'] = '{$a} search area is not available.';
 $string['filesinindexdirectory'] = 'Files in index directory';
diff --git a/search/classes/document.php b/search/classes/document.php
index 97cdc7b93969c..64831b189c9b3 100644
--- a/search/classes/document.php
+++ b/search/classes/document.php
@@ -280,6 +280,16 @@ public function set($fieldname, $value) {
         } else {
             // Replace all groups of line breaks and spaces by single spaces.
             $this->data[$fieldname] = preg_replace("/\s+/u", " ", $value);
+            if ($this->data[$fieldname] === null) {
+                if (isset($this->data['id'])) {
+                    $docid = $this->data['id'];
+                } else {
+                    $docid = '(unknown)';
+                }
+                throw new \moodle_exception('error_indexing', 'search', '', null, '"' . $fieldname .
+                        '" value causes preg_replace error (may be caused by unusual characters) ' .
+                        'in document with id "' . $docid . '"');
+            }
         }
 
         return $this->data[$fieldname];