diff --git a/demo/src/lib/Store.svelte b/demo/src/lib/Store.svelte
index ee90cb0..45094bc 100644
--- a/demo/src/lib/Store.svelte
+++ b/demo/src/lib/Store.svelte
@@ -12,7 +12,7 @@
         }
     });
 
-    const clickHandler = (value = 0) => {
+    const clickHandler = (value: null | number = 0) => {
         store.update(counter => value === null ? 0 : counter + value);
     }
 </script>