diff --git a/backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java b/backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java
index 0fd93860..85cd7d2c 100644
--- a/backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java
+++ b/backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java
@@ -38,6 +38,10 @@ public enum ErrorCode {
     /** A mass import is already in progress; only one can run at a time. 409 */
     IMPORT_ALREADY_RUNNING,
 
+    // --- Thumbnails ---
+    /** A thumbnail backfill is already in progress; only one can run at a time. 409 */
+    THUMBNAIL_BACKFILL_ALREADY_RUNNING,
+
     // --- Invites ---
     /** The invite code does not exist. 404 */
     INVITE_NOT_FOUND,