diff --git a/phpBB/phpbb/template/twig/extension/icon.php b/phpBB/phpbb/template/twig/extension/icon.php
index 07b01eee4b..5376e88973 100644
--- a/phpBB/phpbb/template/twig/extension/icon.php
+++ b/phpBB/phpbb/template/twig/extension/icon.php
@@ -74,10 +74,13 @@ class icon extends \Twig\Extension\AbstractExtension
 			return '';
 		}
 
+		$not_found = false;
+		$source = '';
+
 		switch ($type)
 		{
 			case 'font':
-				$source = '';
+				// Nothing to do here..
 			break;
 
 			case 'iconify':
@@ -86,19 +89,39 @@ class icon extends \Twig\Extension\AbstractExtension
 			break;
 
 			case 'png':
-				$board_url	= defined('PHPBB_USE_BOARD_URL_PATH') && PHPBB_USE_BOARD_URL_PATH;
-				$web_path	= $board_url ? generate_board_url() . '/' : $environment->get_web_root_path();
-				$style_path	= $this->user->style['style_path'];
+				$filesystem	= $environment->get_filesystem();
+				$root_path	= $environment->get_web_root_path();
 
-				$source = "{$web_path}styles/{$style_path}/theme/icons/png/{$icon}.png";
+				$board_url	= defined('PHPBB_USE_BOARD_URL_PATH') && PHPBB_USE_BOARD_URL_PATH;
+				$base_path	= $board_url ? generate_board_url() . '/' : $root_path;
+
+				// Iterate over the user's styles and check for icon existance
+				foreach ($this->get_style_list() as $style_path)
+				{
+					if ($filesystem->exists("{$root_path}styles/{$style_path}/theme/icons/png/{$icon}.png"))
+					{
+						$source = "{$base_path}styles/{$style_path}/theme/icons/png/{$icon}.png";
+
+						break;
+					}
+				}
+
+				// Check if the icon was found or not
+				$not_found = empty($source);
 			break;
 
 			case 'svg':
 				try
 				{
+					// Try to load and prepare the SVG icon
 					$file	= $environment->load('icons/svg/' . $icon . '.svg');
 					$source	= $this->prepare_svg($file);
 				}
+				catch (\Twig\Error\LoaderError $e)
+				{
+					// Icon was not found
+					$not_found = true;
+				}
 				catch (\Twig\Error\Error $e)
 				{
 					return '';
@@ -110,6 +133,20 @@ class icon extends \Twig\Extension\AbstractExtension
 			break;
 		}
 
+		// If no PNG or SVG icon was found, display a default 404 PNG icon.
+		if ($not_found)
+		{
+			if (empty($base_path))
+			{
+				$board_url = defined('PHPBB_USE_BOARD_URL_PATH') && PHPBB_USE_BOARD_URL_PATH;
+				$base_path = $board_url ? generate_board_url() . '/' : $environment->get_web_root_path();
+			}
+
+			$source = "{$base_path}styles/chameleon/theme/icons/png/404.png";
+			$type = 'png';
+			$icon = '404';
+		}
+
 		try
 		{
 			return $environment->render("macros/icons/{$type}.html", [
@@ -130,8 +167,11 @@ class icon extends \Twig\Extension\AbstractExtension
 	/**
 	 * Prepare an SVG for usage in the template icon.
 	 *
+	 * This removes any <?xml ?> and <!DOCTYPE> elements,
+	 * aswell as any <svg> and <title> elements.
+	 *
 	 * @param \Twig\TemplateWrapper	$file	The SVG file loaded from the environment
-	 * @return string
+	 * @return string						The cleaned SVG
 	 */
 	protected function prepare_svg(\Twig\TemplateWrapper $file)
 	{
@@ -195,4 +235,21 @@ class icon extends \Twig\Extension\AbstractExtension
 
 		return $string;
 	}
+
+	/**
+	 * Get the style tree of the style preferred by the current user.
+	 *
+	 * @return array					Style tree, most specific first
+	 */
+	protected function get_style_list()
+	{
+		$style_list = [$this->user->style['style_path']];
+
+		if ($this->user->style['style_parent_id'])
+		{
+			$style_list = array_merge($style_list, array_reverse(explode('/', $this->user->style['style_parent_tree'])));
+		}
+
+		return $style_list;
+	}
 }
diff --git a/tests/template/extension_test.php b/tests/template/extension_test.php
index b46e83c7cd..3a6421da7c 100644
--- a/tests/template/extension_test.php
+++ b/tests/template/extension_test.php
@@ -40,7 +40,15 @@ class phpbb_template_extension_test extends phpbb_template_template_test_case
 		$auth->method('acl_get')
 			->willReturn(true);
 
-		$filesystem = new \phpbb\filesystem\filesystem();
+		$filesystem = $this->createMock('\phpbb\filesystem\filesystem');
+		$filesystem->expects($this->any())
+			->method('exists')
+			->with($this->stringContains('theme/icons/png/'))
+			->will($this->returnValueMap([
+				['phpBB/styles/chameleon/theme/icons/png/phone.png', true],
+				['phpBB/styles/chameleon/theme/icons/png/pencil.png', true],
+				['phpBB/styles/chameleon/theme/icons/png/user.png', false],
+			]));
 		$request = new phpbb_mock_request;
 		$symfony_request = new \phpbb\symfony_request(
 			$request
@@ -365,6 +373,21 @@ class phpbb_template_extension_test extends phpbb_template_template_test_case
 				],
 				'<img class="o-icon o-icon-png png-pencil my-class" src="phpBB/styles/chameleon/theme/icons/png/pencil.png" alt="Pencil icon" data-url="my-test-url/test-page.php?u=2" />',
 			],
+			/** PNG: Not found */
+			[
+				[
+					'type'			=> 'png',
+					'icon'			=> 'user',
+					'title'			=> 'ICON_USER',
+					'hidden'		=> false,
+					'classes'		=> 'my-class',
+					'attributes'	=> [],
+				],
+				[
+					'ICON_USER'		=> 'User icon',
+				],
+				'<img class="o-icon o-icon-png png-404 my-class" src="phpBB/styles/chameleon/theme/icons/png/404.png" alt="User icon" />',
+			],
 			/** SVG: default */
 			[
 				[
@@ -405,6 +428,19 @@ class phpbb_template_extension_test extends phpbb_template_template_test_case
 					<path d="M0 0h24v24H0z" fill="none"></path>
 				</svg>',
 			],
+			/** SVG: Not found */
+			[
+				[
+					'type'			=> 'svg',
+					'icon'			=> 'not-existent',
+					'title'			=> 'Just a title',
+					'hidden'		=> false,
+					'classes'		=> '',
+					'attributes'	=> [],
+				],
+				[],
+				'<img class="o-icon o-icon-png png-404" src="phpBB/styles/chameleon/theme/icons/png/404.png" alt="Just a title" />',
+			],
 			/** SVG: Sanitization */
 			[
 				[