diff --git a/gcn/notices/core/FollowUp.schema.json b/gcn/notices/core/FollowUp.schema.json
index d030b137..773353de 100644
--- a/gcn/notices/core/FollowUp.schema.json
+++ b/gcn/notices/core/FollowUp.schema.json
@@ -19,8 +19,8 @@
     },
     "reference": {
       "type": "object",
-      "additionalProperties": { "type": "number" },
-      "description": "Reference as distributed by the notices or circulars or ATel, ex. gcn.notices.Fermi.gbm.alert: 4642"
+      "additionalProperties": { "type": ["number", "string"] },
+      "description": "Reference as distributed by the notices or circulars or ATel, ex. gcn circulars: 12345, gcn.notices.LVK.alert: SYYMMDDak-2-preliminary. "
     }
   }
 }
diff --git a/validate.mjs b/validate.mjs
index 835b3a78..575225ef 100644
--- a/validate.mjs
+++ b/validate.mjs
@@ -4,7 +4,11 @@ import addFormats from 'ajv-formats'
 import meta from 'ajv/dist/refs/json-schema-2020-12/index.js'
 import { glob as baseGlob } from 'glob'
 
-const ajv = new Ajv({ validateSchema: true, verbose: true })
+const ajv = new Ajv({
+  validateSchema: true,
+  verbose: true,
+  allowUnionTypes: true,
+})
 addFormats(ajv)
 ajv.addMetaSchema(meta)