diff --git a/README.md b/README.md
index 477c22f..c1b7a2c 100644
--- a/README.md
+++ b/README.md
@@ -119,7 +119,7 @@ document will be implicitly polyfilled.
 
 ### roots
 
-type: `(Document | HTMLElement)[]`, default: `[document]`
+type: `(Document | HTMLElement | ShadowRoot)[]`, default: `[document]`
 
 By default the polyfill applies to `document`, but you can configure one or more
 shadow roots the polyfill should apply to using this option. See the
diff --git a/src/polyfill.ts b/src/polyfill.ts
index 88075a4..fc02c44 100644
--- a/src/polyfill.ts
+++ b/src/polyfill.ts
@@ -560,7 +560,7 @@ async function position(rules: AnchorPositions, useAnimationFrame = false) {
   }
 }
 
-export type AnchorPositioningRoot = Document | HTMLElement;
+export type AnchorPositioningRoot = Document | HTMLElement | ShadowRoot;
 
 export interface AnchorPositioningPolyfillOptions {
   // Whether to use `requestAnimationFrame()` when updating target elements’