diff --git a/benchmark/bench.js b/benchmark/bench.js
index acb87b94..cd4bda87 100644
--- a/benchmark/bench.js
+++ b/benchmark/bench.js
@@ -40,6 +40,13 @@ for (let i = 0; i < SHORT_ARRAY_SIZE; i++) {
 }
 
 const benchmarks = [
+  {
+    name: 'empty string',
+    schema: {
+      type: 'string'
+    },
+    input: ''
+  },
   {
     name: 'short string',
     schema: {
diff --git a/lib/serializer.js b/lib/serializer.js
index d8ff4b87..eadaa074 100644
--- a/lib/serializer.js
+++ b/lib/serializer.js
@@ -1,7 +1,7 @@
 'use strict'
 
 // eslint-disable-next-line
-const STR_ESCAPE = /[\u0000-\u001f\u0022\u005c\ud800-\udfff]/
+const ASCII_ESCAPE = /[\u0000-\u001f\u0022\u005c]/
 
 module.exports = class Serializer {
   constructor (options) {
@@ -92,6 +92,7 @@ module.exports = class Serializer {
 
   asString (str) {
     const len = str.length
+
     if (len === 0) {
       return '""'
     } else if (len < 42) {
@@ -113,13 +114,13 @@ module.exports = class Serializer {
           last === -1 && (last = 0)
           result += str.slice(last, i) + '\\'
           last = i
-        } else if (point < 32 || (point >= 0xD800 && point <= 0xDFFF)) {
+        } else if (point < 32 || (point >= 0xd800 && point <= 0xdfff)) {
           // The current character is non-printable characters or a surrogate.
           return JSON.stringify(str)
         }
       }
       return (last === -1 && ('"' + str + '"')) || ('"' + result + str.slice(last) + '"')
-    } else if (len < 5000 && STR_ESCAPE.test(str) === false) {
+    } else if (len < 5000 && str.isWellFormed() && ASCII_ESCAPE.test(str) === false) {
       // Only use the regular expression for shorter input. The overhead is otherwise too much.
       return '"' + str + '"'
     } else {
diff --git a/test/string.test.js b/test/string.test.js
index 518513da..0b3a76c8 100644
--- a/test/string.test.js
+++ b/test/string.test.js
@@ -34,6 +34,21 @@ test('serialize short string', (t) => {
   t.assert.equal(JSON.parse(output), input)
 })
 
+test('serialize medium string', (t) => {
+  t.plan(2)
+
+  const schema = {
+    type: 'string'
+  }
+
+  const input = new Array(150).fill('\x00').join('')
+  const stringify = build(schema)
+  const output = stringify(input)
+
+  t.assert.equal(output, `"${new Array(150).fill('\\u0000').join('')}"`)
+  t.assert.equal(JSON.parse(output), input)
+})
+
 test('serialize long string', (t) => {
   t.plan(2)