diff --git a/src/Error/BaseErrorHandler.php b/src/Error/BaseErrorHandler.php
index aab65ce1121..758dc914e44 100644
--- a/src/Error/BaseErrorHandler.php
+++ b/src/Error/BaseErrorHandler.php
@@ -231,7 +231,7 @@ public function handleFatalError($code, $description, $file, $line)
      * Increases the PHP "memory_limit" ini setting by the specified amount
      * in kilobytes
      *
-     * @param string $additionalKb Number in kilobytes
+     * @param int $additionalKb Number in kilobytes
      * @return void
      */
     public function increaseMemoryLimit($additionalKb)
@@ -242,7 +242,7 @@ public function increaseMemoryLimit($additionalKb)
         }
         $limit = trim($limit);
         $units = strtoupper(substr($limit, -1));
-        $current = substr($limit, 0, strlen($limit) - 1);
+        $current = (int)substr($limit, 0, strlen($limit) - 1);
         if ($units === 'M') {
             $current = $current * 1024;
             $units = 'K';
diff --git a/src/Error/ErrorHandler.php b/src/Error/ErrorHandler.php
index 531d94082d1..fd3c386f6f9 100644
--- a/src/Error/ErrorHandler.php
+++ b/src/Error/ErrorHandler.php
@@ -94,7 +94,7 @@ public function __construct($options = [])
         $defaults = [
             'log' => true,
             'trace' => false,
-            'exceptionRenderer' => 'Cake\Error\ExceptionRenderer',
+            'exceptionRenderer' => ExceptionRenderer::class,
         ];
         $this->_options = $options + $defaults;
     }
@@ -127,13 +127,14 @@ protected function _displayError($error, $debug)
      */
     protected function _displayException($exception)
     {
-        $renderer = App::className($this->_options['exceptionRenderer'], 'Error');
+        $rendererClassName = App::className($this->_options['exceptionRenderer'], 'Error');
         try {
-            if (!$renderer) {
-                throw new Exception("$renderer is an invalid class.");
+            if (!$rendererClassName) {
+                throw new Exception("$rendererClassName is an invalid class.");
             }
-            $error = new $renderer($exception);
-            $response = $error->render();
+            /* @var \Cake\Error\ExceptionRendererInterface $renderer */
+            $renderer = new $rendererClassName($exception);
+            $response = $renderer->render();
             $this->_clearOutput();
             $this->_sendResponse($response);
         } catch (Exception $e) {
diff --git a/src/Error/ExceptionRenderer.php b/src/Error/ExceptionRenderer.php
index 3d7bdf41ea7..1008848ed6b 100644
--- a/src/Error/ExceptionRenderer.php
+++ b/src/Error/ExceptionRenderer.php
@@ -47,7 +47,7 @@
  * Using a subclass of ExceptionRenderer gives you full control over how Exceptions are rendered, you
  * can configure your class in your config/app.php.
  */
-class ExceptionRenderer
+class ExceptionRenderer implements ExceptionRendererInterface
 {
 
     /**
@@ -121,6 +121,7 @@ protected function _getController()
 
         try {
             $class = App::className('Error', 'Controller', 'Controller');
+            /* @var \Cake\Controller\Controller $controller */
             $controller = new $class($request, $response);
             $controller->startupProcess();
             $startup = true;
diff --git a/src/Error/ExceptionRendererInterface.php b/src/Error/ExceptionRendererInterface.php
new file mode 100644
index 00000000000..5535e356ceb
--- /dev/null
+++ b/src/Error/ExceptionRendererInterface.php
@@ -0,0 +1,26 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.4.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Error;
+
+
+interface ExceptionRendererInterface
+{
+    /**
+     * Renders the response for the exception.
+     *
+     * @return \Cake\Network\Response The response to be sent.
+     */
+    public function render();
+}
diff --git a/src/Error/Middleware/ErrorHandlerMiddleware.php b/src/Error/Middleware/ErrorHandlerMiddleware.php
index 84117a7f7ce..e158e73e7ef 100644
--- a/src/Error/Middleware/ErrorHandlerMiddleware.php
+++ b/src/Error/Middleware/ErrorHandlerMiddleware.php
@@ -18,6 +18,7 @@
 use Cake\Core\Configure;
 use Cake\Core\InstanceConfigTrait;
 use Cake\Error\ExceptionRenderer;
+use Cake\Error\ExceptionRendererInterface;
 use Cake\Log\Log;
 use Exception;
 
@@ -55,7 +56,7 @@ class ErrorHandlerMiddleware
     /**
      * Exception render.
      *
-     * @var \Cake\Error\ExceptionRenderer|string|null
+     * @var \Cake\Error\ExceptionRendererInterface|string|null
      */
     protected $exceptionRenderer;
 
@@ -127,7 +128,7 @@ public function handleException($exception, $request, $response)
      * Get a renderer instance
      *
      * @param \Exception $exception The exception being rendered.
-     * @return \Cake\Error\BaseErrorHandler The exception renderer.
+     * @return \Cake\Error\ExceptionRendererInterface The exception renderer.
      * @throws \Exception When the renderer class cannot be found.
      */
     protected function getRenderer($exception)
@@ -139,7 +140,13 @@ protected function getRenderer($exception)
         if (is_string($this->exceptionRenderer)) {
             $class = App::className($this->exceptionRenderer, 'Error');
             if (!$class) {
-                throw new Exception("The '{$this->exceptionRenderer}' renderer class could not be found.");
+                throw new Exception(sprintf("The '%s' renderer class could not be found.",
+                    $this->exceptionRenderer));
+            }
+            if (!$class instanceof ExceptionRendererInterface) {
+                throw new Exception(sprintf("The '%s' renderer class must implement %s.",
+                    $this->exceptionRenderer,
+                    ExceptionRendererInterface::class));
             }
 
             return new $class($exception);