diff --git a/doc/api/util.md b/doc/api/util.md
index 2653c01af1d4c9..9cca721e63cc94 100644
--- a/doc/api/util.md
+++ b/doc/api/util.md
@@ -525,6 +525,16 @@ console.log(promisified === doSomething[util.promisify.custom]);
 This can be useful for cases where the original function does not follow the
 standard format of taking an error-first callback as the last argument.
 
+For example, with a function that takes in `(foo, onSuccessCallback, onErrorCallback)`:
+
+```js
+doSomething[util.promisify.custom] = function(foo) {
+  return new Promise(function(resolve, reject) {
+    doSomething(foo, resolve, reject);
+  });
+};
+```
+
 ### util.promisify.custom
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