diff --git a/fastify.js b/fastify.js
index 7a3829b91e..476c2a51e8 100644
--- a/fastify.js
+++ b/fastify.js
@@ -1,6 +1,6 @@
 'use strict'
 
-const VERSION = '4.2.0'
+const VERSION = '4.2.1'
 
 const Avvio = require('avvio')
 const http = require('http')
diff --git a/lib/error-serializer.js b/lib/error-serializer.js
index 480effc7b7..8e8b4f52a1 100644
--- a/lib/error-serializer.js
+++ b/lib/error-serializer.js
@@ -74,24 +74,24 @@ class Serializer {
     return bool === null ? 'null' : this.asBoolean(bool)
   }
 
-  asDatetime (date) {
-    const quotes = '"'
+  asDateTime (date) {
+    if (date === null) return '""'
     if (date instanceof Date) {
-      return quotes + date.toISOString() + quotes
+      return '"' + date.toISOString() + '"'
     }
-    return this.asString(date)
+    throw new Error(`The value "${date}" cannot be converted to a date-time.`)
   }
 
-  asDatetimeNullable (date) {
-    return date === null ? 'null' : this.asDatetime(date)
+  asDateTimeNullable (date) {
+    return date === null ? 'null' : this.asDateTime(date)
   }
 
   asDate (date) {
-    const quotes = '"'
+    if (date === null) return '""'
     if (date instanceof Date) {
-      return quotes + new Date(date.getTime() - (date.getTimezoneOffset() * 60000)).toISOString().slice(0, 10) + quotes
+      return '"' + new Date(date.getTime() - (date.getTimezoneOffset() * 60000)).toISOString().slice(0, 10) + '"'
     }
-    return this.asString(date)
+    throw new Error(`The value "${date}" cannot be converted to a date.`)
   }
 
   asDateNullable (date) {
@@ -99,11 +99,11 @@ class Serializer {
   }
 
   asTime (date) {
-    const quotes = '"'
+    if (date === null) return '""'
     if (date instanceof Date) {
-      return quotes + new Date(date.getTime() - (date.getTimezoneOffset() * 60000)).toISOString().slice(11, 19) + quotes
+      return '"' + new Date(date.getTime() - (date.getTimezoneOffset() * 60000)).toISOString().slice(11, 19) + '"'
     }
-    return this.asString(date)
+    throw new Error(`The value "${date}" cannot be converted to a time.`)
   }
 
   asTimeNullable (date) {
diff --git a/package.json b/package.json
index 8b9f097d62..0387452b10 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "fastify",
-  "version": "4.2.0",
+  "version": "4.2.1",
   "description": "Fast and low overhead web framework, for Node.js",
   "main": "fastify.js",
   "type": "commonjs",