diff --git a/packages/testkit-backend/src/backend.js b/packages/testkit-backend/src/backend.js
index f1f8983cd..80fc63265 100644
--- a/packages/testkit-backend/src/backend.js
+++ b/packages/testkit-backend/src/backend.js
@@ -25,7 +25,7 @@ export default class Backend {
       try {
         this._controller.handle(contextId, request)
       } catch (e) {
-        this._channel.writeBackendError(contextId, e)
+        this._channel.writeBackendError(contextId, e.message)
       }
     })
   }
diff --git a/packages/testkit-backend/src/channel/testkit-protocol.js b/packages/testkit-backend/src/channel/testkit-protocol.js
index bacc41e64..1670566a3 100644
--- a/packages/testkit-backend/src/channel/testkit-protocol.js
+++ b/packages/testkit-backend/src/channel/testkit-protocol.js
@@ -1,5 +1,6 @@
 import readline from 'readline'
 import EventEmitter from 'events'
+import stringify from '../stringify'
 
 export default class Protocol extends EventEmitter {
   constructor (stream) {
@@ -61,7 +62,7 @@ export default class Protocol extends EventEmitter {
 
   serializeResponse (response) {
     console.log('> writing response', response)
-    const responseStr = this._stringify(response)
+    const responseStr = stringify(response)
     return  ['#response begin', responseStr, '#response end'].join('\n') + '\n'
   }
 
@@ -72,9 +73,4 @@ export default class Protocol extends EventEmitter {
     this.emit('request', { name, data })
   }
   
-  _stringify (val) {
-    return JSON.stringify(val, (_, value) =>
-      typeof value === 'bigint' ? `${value}n` : value
-    )
-  }
 }
diff --git a/packages/testkit-backend/src/controller/local.js b/packages/testkit-backend/src/controller/local.js
index 1c59ede1b..ae2638e51 100644
--- a/packages/testkit-backend/src/controller/local.js
+++ b/packages/testkit-backend/src/controller/local.js
@@ -1,5 +1,6 @@
 import Context from '../context'
 import Controller from './interface'
+import stringify from '../stringify'
 
 
 /**
diff --git a/packages/testkit-backend/src/stringify.js b/packages/testkit-backend/src/stringify.js
new file mode 100644
index 000000000..996e6dad4
--- /dev/null
+++ b/packages/testkit-backend/src/stringify.js
@@ -0,0 +1,5 @@
+export default function stringify(json) {
+  return JSON.stringify(json, (_, value) =>
+    typeof value === 'bigint' ? `${value}n` : value
+  )
+}