diff --git a/lib/private/FullTextSearch/Model/IndexDocument.php b/lib/private/FullTextSearch/Model/IndexDocument.php
index 55cce902a2..d5bef90602 100644
--- a/lib/private/FullTextSearch/Model/IndexDocument.php
+++ b/lib/private/FullTextSearch/Model/IndexDocument.php
@@ -359,11 +359,16 @@ class IndexDocument implements IIndexDocument, JsonSerializable {
 	 * @return IIndexDocument
 	 */
 	final public function addSubTag(string $sub, string $tag): IIndexDocument {
-		$this->subTags[$sub] = $tag;
+		if (!array_key_exists($sub, $this->subTags)) {
+			$this->subTags[$sub] = [];
+		}
+
+		$this->subTags[$sub][] = $tag;
 
 		return $this;
 	}
 
+
 	/**
 	 * Set the list of sub tags assigned to the original document.
 	 *