diff --git a/api/v1/swagger/definitions/patron.json b/api/v1/swagger/definitions/patron.json
index 84f298ae134..c93c4baeb5d 100644
--- a/api/v1/swagger/definitions/patron.json
+++ b/api/v1/swagger/definitions/patron.json
@@ -267,7 +267,7 @@
       "description": "produce a warning for this patron if this item has previously been checked out to this patron if 'yes', not if 'no', defer to category setting if 'inherit'"
     },
     "updated_on": {
-      "type": "string",
+      "type": ["string", "null"],
       "description": "time of last change could be useful for synchronization with external systems (among others)"
     },
     "lastseen": {