assert object type in ctx.onerror

lib/context.js

@@ -3,6 +3,7 @@
  * Module dependencies.
  */
 
+var assert = require('assert');
 var delegate = require('delegates');
 var http = require('http');
 
@@ -86,7 +87,9 @@ var proto = module.exports = {
     // don't do anything if there is no error.
     // this allows you to pass `this.onerror`
     // to node-style callbacks.
-    if (!err) return;
+    if (undefined == err) return;
+
+    assert(err instanceof Error, 'non-error thrown: ' + err);
 
     // nothing we can do here other
     // than delegate to the app-level