diff --git a/lib/context.js b/lib/context.js
index 8d7dfb9..0511558 100644
--- a/lib/context.js
+++ b/lib/context.js
@@ -3,7 +3,7 @@
  * Module dependencies.
  */
 
-var debug = require('debug')('koa:context');
+var assert = require('assert');
 var delegate = require('delegates');
 var http = require('http');
 
@@ -87,7 +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
diff --git a/lib/request.js b/lib/request.js
index 98a6b85..27ad505 100644
--- a/lib/request.js
+++ b/lib/request.js
@@ -3,7 +3,6 @@
  * Module dependencies.
  */
 
-var debug = require('debug')('koa:request');
 var qs = require('querystring');
 var typeis = require('type-is');
 var fresh = require('fresh');
diff --git a/lib/response.js b/lib/response.js
index 4e399c6..b5d0dbc 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -4,7 +4,6 @@
  */
 
 var ensureErrorHandler = require('error-inject');
-var debug = require('debug')('koa:response');
 var escape = require('escape-html');
 var onfinish = require('finished');
 var status = require('statuses');
diff --git a/test/context/onerror.js b/test/context/onerror.js
index 65a9262..6bf9c85 100644
--- a/test/context/onerror.js
+++ b/test/context/onerror.js
@@ -50,4 +50,4 @@ describe('ctx.onerror(err)', function(){
       done();
     })
   })
-})
\ No newline at end of file
+})