diff --git a/doc/api/util.md b/doc/api/util.md
index 510b8c92ac9e5f..c284d023498dfc 100644
--- a/doc/api/util.md
+++ b/doc/api/util.md
@@ -512,8 +512,10 @@ If there is an `original[util.promisify.custom]` property present, `promisify`
 will return its value, see [Custom promisified functions][].
 
 `promisify()` assumes that `original` is a function taking a callback as its
-final argument in all cases, and the returned function will result in undefined
-behavior if it does not.
+final argument in all cases. If `original` is not a function, `promisify()`
+will throw an error. If `original` is a function but its last argument is not a
+Node.js style callback, it will still be passed a Node.js style callback
+as its last argument.
 
 ### Custom promisified functions
 
@@ -548,6 +550,8 @@ doSomething[util.promisify.custom] = (foo) => {
   });
 };
 ```
+If `promisify.custom` is defined but is not a function, `promisify()` will
+throw an error.
 
 ### util.promisify.custom
 <!-- YAML