diff --git a/README.md b/README.md
index 500f0c5c..21fba128 100644
--- a/README.md
+++ b/README.md
@@ -162,6 +162,8 @@ The filter property determines whether or not an element should be replaced with
  * `filter: 'p'` will select `<p>` elements
  * `filter: ['em', 'i']` will select `<em>` or `<i>` elements
 
+The tag names in the `filter` property are expected in lowercase, regardless of their form in the document.
+
 Alternatively, the filter can be a function that returns a boolean depending on whether a given node should be replaced. The function is passed a DOM node as well as the `TurndownService` options. For example, the following rule selects `<a>` elements (with an `href`) when the `linkStyle` option is `inlined`:
 
 ```js