diff --git a/index.js b/index.js
index cd7ba6e0..c73cf32c 100644
--- a/index.js
+++ b/index.js
@@ -231,7 +231,8 @@ function additionalProperty (schema, externalSchema, fullSchema) {
   let code = ''
   if (ap === true) {
     return `
-        json += $asString(keys[i]) + ':' + fastSafeStringify(obj[keys[i]]) + ','
+        if (obj[keys[i]] !== undefined)
+          json += $asString(keys[i]) + ':' + fastSafeStringify(obj[keys[i]]) + ','
     `
   }
   if (ap['$ref']) {
diff --git a/test/additionalProperties.test.js b/test/additionalProperties.test.js
index 4287b5b3..6f9dd502 100644
--- a/test/additionalProperties.test.js
+++ b/test/additionalProperties.test.js
@@ -254,3 +254,20 @@ test('nested additionalProperties set to true', (t) => {
   let obj = { ap: { value: 'string', someNumber: 42 } }
   t.equal('{"ap":{"value":"string","someNumber":42}}', stringify(obj))
 })
+
+test('field passed to fastSafeStringify as undefined should be removed', (t) => {
+  t.plan(1)
+  const stringify = build({
+    title: 'nested additionalProperties=true',
+    type: 'object',
+    properties: {
+      ap: {
+        type: 'object',
+        additionalProperties: true
+      }
+    }
+  })
+
+  let obj = { ap: { value: 'string', someNumber: undefined } }
+  t.equal('{"ap":{"value":"string"}}', stringify(obj))
+})