diff --git a/theme-source/render-to-string.tsx b/theme-source/render-to-string.tsx
index 1003d618a7a82b5ef964d50f9508833fca3a8fda..d0af140cd3b964b909f7c39361707caddbfe9628 100644
--- a/theme-source/render-to-string.tsx
+++ b/theme-source/render-to-string.tsx
@@ -23,5 +23,5 @@ export function renderToString(Component: JSX.Element, htmlProps: any) {
{delimiter}
</Html>,
).split(delimiter)
- return html[0] + content + html[1]
+ return '<!DOCTYPE html>\n' + html[0] + content + html[1]
}