diff --git a/.changeset/beige-kangaroos-appear.md b/.changeset/beige-kangaroos-appear.md
new file mode 100644
index 000000000000..a17d19dc3a81
--- /dev/null
+++ b/.changeset/beige-kangaroos-appear.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] adjust ActionData type
diff --git a/packages/kit/src/core/sync/write_types/index.js b/packages/kit/src/core/sync/write_types/index.js
index e6b13cd94c85..fac2e952b1d4 100644
--- a/packages/kit/src/core/sync/write_types/index.js
+++ b/packages/kit/src/core/sync/write_types/index.js
@@ -401,7 +401,7 @@ function process_node(node, outdir, is_page, proxies, all_pages_have_load = true
 						? `./proxy${replace_ext_with_js(basename)}`
 						: path_to_original(outdir, node.server);
 
-					type = `Expand<Kit.AwaitedActions<typeof import('${from}').actions>> | undefined`;
+					type = `Expand<Kit.AwaitedActions<typeof import('${from}').actions>> | null`;
 				}
 			}
 			exports.push(`export type ActionData = ${type};`);
diff --git a/packages/kit/src/runtime/client/client.js b/packages/kit/src/runtime/client/client.js
index 3957e2d57439..4d567052ea5e 100644
--- a/packages/kit/src/runtime/client/client.js
+++ b/packages/kit/src/runtime/client/client.js
@@ -497,7 +497,7 @@ export function create_client({ target, base }) {
 				route,
 				status,
 				url: new URL(url),
-				form,
+				form: form ?? null,
 				// The whole page store is updated, but this way the object reference stays the same
 				data: data_changed ? data : page.data
 			};