diff --git a/tests/Helper/Stringer.php b/tests/Helper/Stringer.php
new file mode 100644
index 000000000..b36330323
--- /dev/null
+++ b/tests/Helper/Stringer.php
@@ -0,0 +1,40 @@
+<?php
+
+namespace Behat\Mink\Tests\Helper;
+
+/**
+ * A container class to hold a string.
+ *
+ * @package Behat\Mink\Tests\helpers
+ */
+class Stringer
+{
+
+    /**
+     * Internal storage.
+     *
+     * @var string
+     */
+    private $content;
+
+    /**
+     * Stringer constructor.
+     *
+     * @param string $content
+     */
+    public function __construct($content)
+    {
+        $this->content = $content;
+    }
+
+    /**
+     * Returns the wrapped string.
+     *
+     * @return string
+     */
+    public function __toString()
+    {
+        return $this->content;
+    }
+
+}
diff --git a/tests/WebAssertTest.php b/tests/WebAssertTest.php
index 92f3a1e10..f9a52c6d8 100644
--- a/tests/WebAssertTest.php
+++ b/tests/WebAssertTest.php
@@ -3,7 +3,7 @@
 namespace Behat\Mink\Tests;
 
 use Behat\Mink\Exception\ExpectationException;
-use Behat\Mink\Tests\helpers\Stringer;
+use Behat\Mink\Tests\Helper\Stringer;
 use Behat\Mink\WebAssert;
 use PHPUnit\Framework\TestCase;
 
@@ -256,7 +256,7 @@ public function testResponseHeaderContains()
                 )
             ));
 
-        $this->assertCorrectAssertion('responseHeaderContains', array('foo', new Stringer('ba')));
+        $this->assertCorrectAssertion('responseHeaderContains', array('foo', 'ba'));
         $this->assertWrongAssertion(
             'responseHeaderContains',
             array('bar', 'bz'),
@@ -277,6 +277,50 @@ public function testResponseHeaderNotContains()
                 )
             ));
 
+        $this->assertCorrectAssertion('responseHeaderNotContains', array('foo', 'bz'));
+        $this->assertWrongAssertion(
+            'responseHeaderNotContains',
+            array('bar', 'ba'),
+            'Behat\\Mink\\Exception\\ExpectationException',
+            'The text "ba" was found in the "bar" response header, but it should not.'
+        );
+    }
+
+    public function testResponseHeaderContainsObjectWithToString()
+    {
+        $this->session
+            ->expects($this->any())
+            ->method('getResponseHeader')
+            ->will($this->returnValueMap(
+              array(
+                array('foo', 'bar'),
+                array('bar', 'baz'),
+              )
+            ));
+
+        $this->assertCorrectAssertion('responseHeaderContains', array('foo', new Stringer('ba')));
+        $this->assertWrongAssertion(
+            'responseHeaderContains',
+            array('bar', 'bz'),
+            'Behat\\Mink\\Exception\\ExpectationException',
+            'The text "bz" was not found anywhere in the "bar" response header.'
+        );
+    }
+
+    public function testResponseHeaderNotContainsObjectWithToString()
+    {
+        $this->session
+            ->expects($this->any())
+            ->method('getResponseHeader')
+            ->will(
+                $this->returnValueMap(
+                    array(
+                        array('foo', 'bar'),
+                        array('bar', 'baz'),
+                    )
+                )
+            );
+
         $this->assertCorrectAssertion('responseHeaderNotContains', array('foo', new Stringer('bz')));
         $this->assertWrongAssertion(
             'responseHeaderNotContains',
@@ -459,7 +503,7 @@ public function testResponseContains()
             ->will($this->returnValue('Some page text'))
         ;
 
-        $this->assertCorrectAssertion('responseContains', array(new Stringer('PAGE text')));
+        $this->assertCorrectAssertion('responseContains', array('PAGE text'));
         $this->assertWrongAssertion(
             'responseContains',
             array('html text'),
@@ -487,6 +531,62 @@ public function testResponseNotContains()
             ->will($this->returnValue('Some html text'))
         ;
 
+        $this->assertCorrectAssertion('responseNotContains', array('PAGE text'));
+        $this->assertWrongAssertion(
+            'responseNotContains',
+            array('HTML text'),
+            'Behat\\Mink\\Exception\\ExpectationException',
+            'The string "HTML text" appears in the HTML response of this page, but it should not.'
+        );
+    }
+
+    public function testResponseContainsObjectWithToString()
+    {
+        $page = $this->getMockBuilder('Behat\\Mink\\Element\\DocumentElement')
+            ->disableOriginalConstructor()
+            ->getMock()
+        ;
+
+        $this->session
+            ->expects($this->exactly(2))
+            ->method('getPage')
+            ->will($this->returnValue($page))
+        ;
+
+        $page
+            ->expects($this->exactly(2))
+            ->method('getContent')
+            ->will($this->returnValue('Some page text'))
+        ;
+
+        $this->assertCorrectAssertion('responseContains', array(new Stringer('PAGE text')));
+        $this->assertWrongAssertion(
+            'responseContains',
+            array('html text'),
+            'Behat\\Mink\\Exception\\ExpectationException',
+            'The string "html text" was not found anywhere in the HTML response of the current page.'
+        );
+    }
+
+    public function testResponseNotContainsObjectWithToString()
+    {
+        $page = $this->getMockBuilder('Behat\\Mink\\Element\\DocumentElement')
+            ->disableOriginalConstructor()
+            ->getMock()
+        ;
+
+        $this->session
+            ->expects($this->exactly(2))
+            ->method('getPage')
+            ->will($this->returnValue($page))
+        ;
+
+        $page
+            ->expects($this->exactly(2))
+            ->method('getContent')
+            ->will($this->returnValue('Some html text'))
+        ;
+
         $this->assertCorrectAssertion('responseNotContains', array(new Stringer('PAGE text')));
         $this->assertWrongAssertion(
             'responseNotContains',
diff --git a/tests/helpers/Stringer.php b/tests/helpers/Stringer.php
deleted file mode 100644
index 34eee5704..000000000
--- a/tests/helpers/Stringer.php
+++ /dev/null
@@ -1,42 +0,0 @@
-<?php
-
-namespace Behat\Mink\Tests\helpers;
-
-/**
- * Class Stringer
- *
- * A container class to hold a string.
- *
- * @package Behat\Mink\Tests\helpers
- */
-class Stringer
-{
-
-  /**
-   * Internal storage.
-   *
-   * @var string
-   */
-  protected $content;
-
-  /**
-   * Stringer constructor.
-   *
-   * @param $content
-   */
-  public function __construct($content)
-  {
-    $this->content = $content;
-  }
-
-  /**
-   * Returns in the wrapped string.
-   *
-   * @return string
-   */
-  public function __toString()
-  {
-    return $this->content;
-  }
-
-}