diff --git a/helpers/middlewares.js b/helpers/middlewares.js
index e3f3fa5..4f77958 100644
--- a/helpers/middlewares.js
+++ b/helpers/middlewares.js
@@ -1,19 +1,20 @@
 exports.exceptionMiddleware = async function exceptionMiddleware(err, req, res, next) {
-    if (!err.contextId) {
-        err.contextId = req.query.contextId;
-        err.pageId = req.query.pageId;
+    if (res.headersSent) {
+        return next(err);
     }
 
-    if (!err.contextId) {
-        next(err);
-    }
+    const contextId = err.contextId || req.query.contextId;
+    const pageId = err.pageId || req.query.pageId;
+    const errorMessage = err.message || 'Unknown error';
 
     res.status(500);
-    res.header('scrapy-puppeteer-service-context-id', err.contextId);
+    if (contextId) {
+        res.header('scrapy-puppeteer-service-context-id', contextId);
+    }
     res.send({
-        'contextId': err.contextId,
-        'pageId': err.pageId,
-        'error_msg': err.message,
+        contextId,
+        pageId,
+        error: errorMessage
     });
     next(err);
 }
diff --git a/helpers/utils.js b/helpers/utils.js
index 784a45e..0825835 100644
--- a/helpers/utils.js
+++ b/helpers/utils.js
@@ -7,7 +7,7 @@ async function findContextInBrowser(browser, contextId) {
             return context;
         }
     }
-    throw "Context not found";
+    throw new Error("Context not found");
 }
 
 async function findPageInContext(context, pageId) {
@@ -16,7 +16,7 @@ async function findPageInContext(context, pageId) {
             return page;
         }
     }
-    throw "Page not found";
+    throw new Error("Page not found");
 }
 
 exports.closeContexts = async function closeContexts(browser, contextIds) {
@@ -48,7 +48,7 @@ async function wait(page, waitFor) {
     }
 
     if ([selector, xpath, timeout].filter(Boolean).length > 1) {
-        throw "Wait options must contain either a selector, an xpath or a timeout";
+        throw new Error("Wait options must contain either a selector, an xpath or a timeout");
     }
 
     if (selector) {
@@ -173,8 +173,7 @@ exports.performAction = async function performAction(request, action) {
 
         try {
             return await action(page, request);
-        }
-        catch (err) {
+        } catch (err) {
             err.contextId = page.browserContext().id;
             err.pageId = page.target()._targetId;
             throw err;
diff --git a/package.json b/package.json
index 117d1b9..6d93c9f 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "scrapy-puppeteer-service",
-  "version": "0.2.2",
+  "version": "0.2.3",
   "private": true,
   "scripts": {
     "start": "node ./bin/www"