diff --git a/.changeset/cuddly-melons-taste.md b/.changeset/cuddly-melons-taste.md
new file mode 100644
index 000000000000..830bbd66d5f1
--- /dev/null
+++ b/.changeset/cuddly-melons-taste.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: allow whitespace in legacy onMount check
diff --git a/packages/kit/src/runtime/client/state.svelte.js b/packages/kit/src/runtime/client/state.svelte.js
index 5e1978869ca3..904be39bd0c6 100644
--- a/packages/kit/src/runtime/client/state.svelte.js
+++ b/packages/kit/src/runtime/client/state.svelte.js
@@ -12,7 +12,7 @@ export let updated;
 
 // this is a bootleg way to tell if we're in old svelte or new svelte
 const is_legacy =
-	onMount.toString().includes('$$') || /function \w+\(\) \{\}/.test(onMount.toString());
+	onMount.toString().includes('$$') || /function \w+\(\) \{\s?\}/.test(onMount.toString());
 
 if (is_legacy) {
 	page = {