diff --git a/packages/react-resizable-panels/src/Panel.ts b/packages/react-resizable-panels/src/Panel.ts
index e94d9d84f..e932580ef 100644
--- a/packages/react-resizable-panels/src/Panel.ts
+++ b/packages/react-resizable-panels/src/Panel.ts
@@ -19,7 +19,7 @@ export type PanelOnCollapse = () => void;
 export type PanelOnExpand = () => void;
 export type PanelOnResize = (
   mixedSizes: MixedSizes,
-  prevMixedSizes: MixedSizes
+  prevMixedSizes: MixedSizes | undefined
 ) => void;
 
 export type PanelCallbacks = {