diff --git a/doc/api/buffer.md b/doc/api/buffer.md
index 7035fa254e77d9..e90d2f2616f8bd 100644
--- a/doc/api/buffer.md
+++ b/doc/api/buffer.md
@@ -528,6 +528,10 @@ Returns the actual byte length of a string. This is not the same as
 [`String.prototype.length`][] since that returns the number of *characters* in
 a string.
 
+*Note* that for `'base64'` and `'hex'`, this function assumes valid input. For
+strings that contain non-Base64/Hex-encoded data (e.g. whitespace), the return
+value might be greater than the length of a `Buffer` created from the string.
+
 Example:
 
 ```js