diff --git a/lib/response.js b/lib/response.js
index 5ef6708..892f430 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -274,7 +274,7 @@ module.exports = {
    */
 
   set type(type) {
-    this.set('Content-Type', getType(type));
+    this.set('Content-Type', getType(type) || 'application/octet-stream');
   },
 
   /**
diff --git a/test/response/type.js b/test/response/type.js
index 1897d87..6fc65b4 100644
--- a/test/response/type.js
+++ b/test/response/type.js
@@ -38,6 +38,15 @@ describe('ctx.type=', function(){
       ctx.response.header['content-type'].should.equal('text/html; charset=foo');
     })
   })
+
+  describe('with an unknown extension', function(){
+    it('should default to application/octet-stream',function(){
+      var ctx = context();
+      ctx.type = 'asdf';
+      ctx.type.should.equal('application/octet-stream');
+      ctx.response.header['content-type'].should.equal('application/octet-stream');
+    })
+  })
 })
 
 describe('ctx.type', function(){