diff --git a/src/Data/YAML/Foreign/Encode.js b/src/Data/YAML/Foreign/Encode.js
index 4b12a1c..8c42980 100644
--- a/src/Data/YAML/Foreign/Encode.js
+++ b/src/Data/YAML/Foreign/Encode.js
@@ -13,14 +13,15 @@ export function objToHash(valueToYAMLImpl, fst, snd, obj) {
 }
 
 export function toYAMLImpl(a) {
-  const replacer = (_, v) => {
-    if (typeof v === 'object') {
-      Object.keys(v).forEach(k => {
-        if (v[k] === null) {
-          delete v[k]
+  const replacer = (_, o) => {
+    if (typeof o === 'object') {
+      Object.entries(o).forEach(([k, v]) => {
+        if (typeof v === 'undefined' || v === null) {
+          delete o[k]
         }
       })
     }
+    return o
   }
 
   return yaml.dump(a, { noCompatMode: true, replacer })