diff --git a/src/useSize.tsx b/src/useSize.tsx
index c488e767..4079dbf4 100644
--- a/src/useSize.tsx
+++ b/src/useSize.tsx
@@ -45,7 +45,7 @@ const useSize = (
   };
 
   useEffect(() => {
-    const iframe: HTMLIFrameElement = ref.current;
+    const iframe: HTMLIFrameElement | null = ref.current;
 
     if(!iframe) {
       // iframe will be undefined if component is already unmounted