diff --git a/doc/api/util.md b/doc/api/util.md
index 033224ba0cc16c..26b74b2d6eb9a1 100644
--- a/doc/api/util.md
+++ b/doc/api/util.md
@@ -552,8 +552,10 @@ 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-style callback, it will be still be passed a node-style callback as its last
+argument anyways.
 
 ### Custom promisified functions
 
@@ -588,6 +590,8 @@ doSomething[util.promisify.custom] = (foo) => {
   });
 };
 ```
+If `promisify.custom` is defined but is not a function, `promisify()` will 
+throw an error.
 
 ### util.promisify.custom
 <!-- YAML