diff --git a/src/index.ts b/src/index.ts
index 7d7e41c..ad0bd13 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -66,7 +66,7 @@ function stringifyString (string: string): string {
   // eslint-disable-next-line no-control-regex
   return '"' + string.replace(/[\\"\u0000-\u001F\u2028\u2029]/g, (m) => {
     if (escMap[m] !== undefined) return escMap[m]
-    return '\\u' + (m.charCodeAt(0) + 0x10000).toString(16).substr(1)
+    return '\\u' + (m.charCodeAt(0) + 0x10000).toString(16).slice(1)
   }) + '"'
 }
 
diff --git a/test/index.test.ts b/test/index.test.ts
index f72e6bc..851f807 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -119,8 +119,8 @@ describe('index.js', function () {
     it('should escape serialized strings', function () {
       const result: string = index(function (a: string, b: string) {
         return a + ' ' + b
-      }, 'foo"bar\\"\n', 'baz')
-      if (!result.includes(')("foo\\"bar\\\\\\"\\n", "baz")')) {
+      }, 'foo"bar\\"\n-\u0000\u2028\u2029\u0007', 'baz')
+      if (!result.includes(')("foo\\"bar\\\\\\"\\n-\\u0000\\u2028\\u2029\\u0007", "baz")')) {
         throw new Error('incorrect serialization: ' + result)
       }
     })