diff --git a/index.js b/index.js
index 45a437e..8e069d5 100644
--- a/index.js
+++ b/index.js
@@ -81,6 +81,8 @@ module.exports = function(serverConfig, sharedConfig, clientStreamHandler){
       server = createWebSocketServer(clientStreamHandler);
     } else if(config.protocol == 'wss') {
       server = createSecureWebSocketServer(config.ssl, clientStreamHandler);
+    } else {
+      throw new Error('negative attach and unknow protocol');
     }
 
     server._css_host = config.host;
diff --git a/tests.js b/tests.js
index 9097785..787859e 100644
--- a/tests.js
+++ b/tests.js
@@ -166,4 +166,24 @@ describe('create-stream-server', function(){
       });
     });
   });
+
+  it('should throw error if negative attach and unknow protocol', function(done) {
+    try {
+      var servers = css({
+        s1: {
+          attach: null,
+          protocol: 'unknow-protocol',
+          host: 'localhost',
+          port: 9010
+        }
+      }, function() {});
+    } catch(err) {
+      assert.deepStrictEqual(
+        err,
+        new Error('negative attach and unknow protocol'),
+        'negative attach and unknow protocol should error'
+      );
+      done();
+    }
+  })
 });