diff --git a/.changeset/six-rockets-jump.md b/.changeset/six-rockets-jump.md
new file mode 100644
index 000000000..c03f87c49
--- /dev/null
+++ b/.changeset/six-rockets-jump.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix CSS scanning bug that could lead to infinite loops
diff --git a/packages/astro/src/core/ssr/css.ts b/packages/astro/src/core/ssr/css.ts
index 550af10b7..15898cdab 100644
--- a/packages/astro/src/core/ssr/css.ts
+++ b/packages/astro/src/core/ssr/css.ts
@@ -18,6 +18,9 @@ export function getStylesForURL(filePath: URL, viteServer: vite.ViteDevServer):
   function crawlCSS(entryModule: string, scanned = new Set<string>()) {
     const moduleName = idToModuleMap.get(entryModule);
     if (!moduleName) return;
+    if (!moduleName.id) return;
+    // mark the entrypoint as scanned to avoid an infinite loop
+    scanned.add(moduleName.id)
     for (const importedModule of moduleName.importedModules) {
       if (!importedModule.id || scanned.has(importedModule.id)) continue;
       const ext = path.extname(importedModule.id.toLowerCase());