diff --git a/packages/vite/src/node/ssr/ssrModuleLoader.ts b/packages/vite/src/node/ssr/ssrModuleLoader.ts
index e1dd392a138aed..e856b02402b659 100644
--- a/packages/vite/src/node/ssr/ssrModuleLoader.ts
+++ b/packages/vite/src/node/ssr/ssrModuleLoader.ts
@@ -195,7 +195,7 @@ async function instantiateModule(
       ssrImportKey,
       ssrDynamicImportKey,
       ssrExportAllKey,
-      result.code + `\n//# sourceURL=${mod.url}`
+      '"use strict";' + result.code + `\n//# sourceURL=${mod.url}`
     )
     await initModule(
       context.global,