diff --git a/controller/admin_controller.php b/controller/admin_controller.php
index a6df3dd..5aa4578 100644
--- a/controller/admin_controller.php
+++ b/controller/admin_controller.php
@@ -159,7 +159,7 @@ public function add_prefix()
 			{
 				$this->trigger_message('TOPIC_PREFIXES_INPUT_EMPTY', E_USER_WARNING);
 			}
-			
+
 			$prefix = $this->manager->add_prefix($tag, $this->forum_id);
 			$this->log($prefix['prefix_tag'], 'ACP_LOG_PREFIX_ADDED');
 		}
diff --git a/tests/controller/add_fails_test.php b/tests/controller/add_fails_test.php
deleted file mode 100644
index 2d51243..0000000
--- a/tests/controller/add_fails_test.php
+++ /dev/null
@@ -1,68 +0,0 @@
-<?php
-/**
- *
- * Topic Prefixes extension for the phpBB Forum Software package.
- *
- * @copyright (c) 2016 phpBB Limited <https://www.phpbb.com>
- * @license GNU General Public License, version 2 (GPL-2.0)
- *
- */
-
-namespace phpbb\topicprefixes\controller;
-
-/**
- * We need to manually include the base file because auto loading won't work since the base
- * uses the same namespace as the controller being tested (to override global scope functions).
- */
-require_once __DIR__ . '/admin_controller_base.php';
-
-class add_fails_test extends admin_controller_base
-{
-	/**
-	 * Data for test_add_prefix
-	 *
-	 * @return array
-	 */
-	public function data_add_prefix()
-	{
-		return array(
-			array(true, true),
-			array(true, false),
-			array(false, false),
-		);
-	}
-
-	/**
-	 * Test add_prefix()
-	 *
-	 * @dataProvider data_add_prefix
-	 * @param $submit
-	 * @param $valid_form
-	 */
-
-	public function test_add_fails($input)
-		{
-			self::$valid_form = $valid_form;
-
-			$this->request->expects(static::once())
-				->method('is_set_post')
-				->willReturn(true);
-			$this->manager->expects(static::once())
-				->method('add_prefix')
-				->willReturn(['prefix_tag' => $input]);
-			$this->log->expects($input ? static::once() : static::never())
-				->method('add');
-			$this->db->expects($input ? static::once() : static::never())
-				->method('sql_fetchrow');
-
-			if ($input == '')
-			{
-				$exceptionName = version_compare(PHP_VERSION, '8.0', '<') ? \PHPUnit\Framework\Error\Error::class : \PHPUnit\Framework\Error\Warning::class;
-				$errno = version_compare(PHP_VERSION, '8.0', '<') ? E_USER_WARNING : E_WARNING;
-				$this->expectExceptionMessage('TOPIC_PREFIXES_INPUT_EMPTY');
-				$this->expectException($exceptionName);
-				$this->expectExceptionCode($errno);
-			}
-
-			$this->controller->add_prefix();
-		}
diff --git a/tests/controller/add_prefix_test.php b/tests/controller/add_prefix_test.php
index 09978bd..bad7898 100644
--- a/tests/controller/add_prefix_test.php
+++ b/tests/controller/add_prefix_test.php
@@ -25,21 +25,25 @@ class add_prefix_test extends admin_controller_base
 	 */
 	public function data_add_prefix()
 	{
-		return array(
-			array(true, true),
-			array(true, false),
-			array(false, false),
-		);
+		return [
+			['topic_prefix1', true, true], // valid prefix, form is being submitted, form is valid
+			['topic_prefix2', true, false], // valid prefix, form is being submitted, form is invalid
+			['topic_prefix3', false, false], // valid prefix, form is not being submitted, form is invalid
+			['', true, true], // invalid prefix, form is being submitted, form is valid
+			['', true, false], // invalid prefix, form is being submitted, form is invalid
+			['', false, true], // invalid prefix, form is not being submitted, form is invalid
+		];
 	}
 
 	/**
 	 * Test add_prefix()
 	 *
 	 * @dataProvider data_add_prefix
+	 * @param $prefix_id
 	 * @param $submit
 	 * @param $valid_form
 	 */
-	public function test_add_prefix($submit, $valid_form)
+	public function test_add_prefix($prefix_id, $submit, $valid_form)
 	{
 		if ($submit)
 		{
@@ -52,20 +56,38 @@ public function test_add_prefix($submit, $valid_form)
 			if (!$valid_form)
 			{
 				// Throws E_WARNING in PHP 8.0+ and E_USER_WARNING in earlier versions
-				$exceptionName = version_compare(PHP_VERSION, '8.0', '<') ? \PHPUnit\Framework\Error\Error::class : \PHPUnit\Framework\Error\Warning::class;
-				$errno = version_compare(PHP_VERSION, '8.0', '<') ? E_USER_WARNING : E_WARNING;
+				$exceptionName = PHP_VERSION_ID < 80000 ? \PHPUnit\Framework\Error\Error::class : \PHPUnit\Framework\Error\Warning::class;
+				$errno = PHP_VERSION_ID < 80000 ? E_USER_WARNING : E_WARNING;
+				$this->expectExceptionMessage($this->language->lang('FORM_INVALID'));
 				$this->expectException($exceptionName);
 				$this->expectExceptionCode($errno);
 			}
 			else
 			{
-				$this->manager->expects(static::once())
+				$valid_prefix = $prefix_id !== '';
+
+				$this->request->expects(static::once())
+					->method('variable')
+					->willReturnMap([
+						['prefix_tag', '', true, \phpbb\request\request_interface::REQUEST, $prefix_id],
+					]);
+
+				if (!$valid_prefix) // Empty string prefixes should trigger error
+				{
+					// Throws E_WARNING in PHP 8.0+ and E_USER_WARNING in earlier versions
+					$exceptionName = PHP_VERSION_ID < 80000 ? \PHPUnit\Framework\Error\Error::class : \PHPUnit\Framework\Error\Warning::class;
+					$errno = PHP_VERSION_ID < 80000 ? E_USER_WARNING : E_WARNING;
+					$this->expectExceptionMessage($this->language->lang('TOPIC_PREFIXES_INPUT_EMPTY'));
+					$this->expectException($exceptionName);
+					$this->expectExceptionCode($errno);
+				}
+				$this->manager->expects($valid_prefix ? static::once() : static::never())
 					->method('add_prefix')
-					->willReturn(['prefix_tag' => 'topic_prefix']);
-				$this->log->expects(static::once())
+					->willReturn(['prefix_tag' => $prefix_id]);
+				$this->log->expects($valid_prefix ? static::once() : static::never())
 					->method('add')
-					->with('admin', static::anything(), static::anything(), 'ACP_LOG_PREFIX_ADDED', static::anything(), ['topic_prefix', 'Test Forum']);
-				$this->db->expects(static::once())
+					->with('admin', static::anything(), static::anything(), 'ACP_LOG_PREFIX_ADDED', static::anything(), [$prefix_id, 'Test Forum']);
+				$this->db->expects($valid_prefix ? static::once() : static::never())
 					->method('sql_fetchrow')
 					->willReturn(['forum_name' => 'Test Forum']);
 			}