missing lang string

public/language/en_GB/error.json

@@ -82,6 +82,8 @@
 	"topic-already-deleted": "This topic has already been deleted",
 	"topic-already-restored": "This topic has already been restored",
 
+	"cant-purge-main-post": "You can't purge the main post, please delete the topic instead",
+
 	"topic-thumbnails-are-disabled": "Topic thumbnails are disabled.",
 	"invalid-file": "Invalid File",
 	"uploads-are-disabled": "Uploads are disabled",