diff --git a/.changeset/tame-hats-give.md b/.changeset/tame-hats-give.md
new file mode 100644
index 000000000..c4158a46b
--- /dev/null
+++ b/.changeset/tame-hats-give.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix crash with unexpected file types in pages directory
diff --git a/packages/astro/src/core/ssr/routing.ts b/packages/astro/src/core/ssr/routing.ts
index 1eedfb67f..d3d20c2d1 100644
--- a/packages/astro/src/core/ssr/routing.ts
+++ b/packages/astro/src/core/ssr/routing.ts
@@ -86,6 +86,7 @@ interface Item {
 export function createRouteManifest({ config, cwd }: { config: AstroConfig; cwd?: string }, logging: LogOptions): ManifestData {
   const components: string[] = [];
   const routes: RouteData[] = [];
+  const validExtensions: Set<string> = new Set(['.astro', '.md']);
 
   function walk(dir: string, parentSegments: Part[][], parentParams: string[]) {
     let items: Item[] = [];
@@ -104,7 +105,7 @@ export function createRouteManifest({ config, cwd }: { config: AstroConfig; cwd?
         return;
       }
       // filter out "foo.astro_tmp" files, etc
-      if (!isDir && !/^(\.[a-z0-9]+)+$/i.test(ext)) {
+      if (!isDir && !validExtensions.has(ext)) {
         return;
       }
       const segment = isDir ? basename : name;
diff --git a/packages/astro/test/fixtures/route-manifest/invalid-extension/about.astro b/packages/astro/test/fixtures/route-manifest/invalid-extension/about.astro
new file mode 100644
index 000000000..e69de29bb
diff --git a/packages/astro/test/fixtures/route-manifest/invalid-extension/image.svg b/packages/astro/test/fixtures/route-manifest/invalid-extension/image.svg
new file mode 100644
index 000000000..e69de29bb
diff --git a/packages/astro/test/fixtures/route-manifest/invalid-extension/index.astro b/packages/astro/test/fixtures/route-manifest/invalid-extension/index.astro
new file mode 100644
index 000000000..e69de29bb
diff --git a/packages/astro/test/fixtures/route-manifest/invalid-extension/styles.css b/packages/astro/test/fixtures/route-manifest/invalid-extension/styles.css
new file mode 100644
index 000000000..e69de29bb
diff --git a/packages/astro/test/route-manifest.test.js b/packages/astro/test/route-manifest.test.js
index 1f32ee6af..8ae58c1d6 100644
--- a/packages/astro/test/route-manifest.test.js
+++ b/packages/astro/test/route-manifest.test.js
@@ -184,6 +184,27 @@ describe('route manifest', () => {
     ]);
   });
 
+ it('ignores invalid route extensions', () => {
+    const { routes } = create('invalid-extension', 'always');
+    expect(cleanRoutes(routes)).to.deep.equal([
+      {
+        type: 'page',
+        pattern: /^\/$/,
+        params: [],
+        component: 'invalid-extension/index.astro',
+        pathname: '/',
+      },
+
+      {
+        type: 'page',
+        pattern: /^\/about\/$/,
+        params: [],
+        component: 'invalid-extension/about.astro',
+        pathname: '/about',
+      },
+    ]);
+  });
+
   it('allows multiple slugs', () => {
     const { routes } = create('multiple-slugs', 'always');