diff --git a/src/Util/ACLFormatter.php b/src/Util/ACLFormatter.php
index d194e2e67e..a7d851508d 100644
--- a/src/Util/ACLFormatter.php
+++ b/src/Util/ACLFormatter.php
@@ -14,13 +14,13 @@ final class ACLFormatter
 	 *
 	 * @param string|null $ids A angle-bracketed list of IDs
 	 *
-	 * @return array|null The array based on the IDs (null in case there is no list)
+	 * @return array The array based on the IDs (empty in case there is no list)
 	 */
 	public function expand(string $ids = null)
 	{
-		// In case there is no ID list, return null (=> no ACL set)
+		// In case there is no ID list, return empty array (=> no ACL set)
 		if (!isset($ids)) {
-			return null;
+			return [];
 		}
 
 		// turn string array of angle-bracketed elements into numeric array
diff --git a/tests/src/Util/ACLFormaterTest.php b/tests/src/Util/ACLFormaterTest.php
index 630a30e40b..ae875856ac 100644
--- a/tests/src/Util/ACLFormaterTest.php
+++ b/tests/src/Util/ACLFormaterTest.php
@@ -12,155 +12,96 @@ use PHPUnit\Framework\TestCase;
  */
 class ACLFormaterTest extends TestCase
 {
-	/**
-	 * test expand_acl, perfect input
-	 */
-	public function testExpandAclNormal()
+	public function assertAcl($text, array $assert = [])
 	{
 		$aclFormatter = new ACLFormatter();
 
-		$text='<1><2><3><' . Group::FOLLOWERS . '><' . Group::MUTUALS . '>';
-		$this->assertEquals(array('1', '2', '3', Group::FOLLOWERS, Group::MUTUALS), $aclFormatter->expand($text));
+		$acl = $aclFormatter->expand($text);
+
+		$this->assertEquals($assert, $acl);
+
+		$this->assertMergable($acl);
+	}
+
+	public function assertMergable(array $aclOne, array $aclTwo = [])
+	{
+		$this->assertTrue(is_array($aclOne));
+		$this->assertTrue(is_array($aclTwo));
+
+		$aclMerged = array_unique(array_merge($aclOne, $aclTwo));
+		$this->assertTrue(is_array($aclMerged));
+
+		return $aclMerged;
+	}
+
+	public function dataExpand()
+	{
+		return [
+			'normal' => [
+				'input' => '<1><2><3><' . Group::FOLLOWERS . '><' . Group::MUTUALS . '>',
+				'assert' => ['1', '2', '3', Group::FOLLOWERS, Group::MUTUALS],
+			],
+			'nigNumber' => [
+				'input' => '<1><' . PHP_INT_MAX . '><15>',
+				'assert' => ['1', (string)PHP_INT_MAX, '15'],
+			],
+			'string' => [
+				'input' => '<1><279012><tt>',
+				'assert' => ['1', '279012'],
+			],
+			'space' => [
+				'input' => '<1><279 012><32>',
+				'assert' => ['1', '32'],
+			],
+			'empty' => [
+				'input' => '',
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'noBrackets' => [
+				'input' => 'According to documentation, that\'s invalid. ', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'justOneBracket' => [
+				'input' => '<Another invalid string', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'justOneBracket2' => [
+				'input' => 'Another invalid> string', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'closeOnly' => [
+				'input' => 'Another> invalid> string>', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'openOnly' => [
+				'input' => '<Another< invalid string<', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception?
+			'noMatching1' => [
+				'input' => '<Another<> invalid <string>', //should be invalid
+				'assert' => [],
+			],
+			/// @todo should there be an exception? Or array(1, 3)
+			//  (This should be array(1,3) - mike)
+			'emptyMatch' => [
+				'input' => '<1><><3>', //should be invalid
+				'assert' => ['1', '3'],
+			],
+		];
 	}
 
 	/**
-	 * test with a big number
+	 * @dataProvider dataExpand
 	 */
-	public function testExpandAclBigNumber()
+	public function testExpand($input, array $assert)
 	{
-		$aclFormatter = new ACLFormatter();
-
-		$text='<1><' . PHP_INT_MAX . '><15>';
-		$this->assertEquals(array('1', (string)PHP_INT_MAX, '15'), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test with a string in it.
-	 *
-	 * @todo is this valid input? Otherwise: should there be an exception?
-	 */
-	public function testExpandAclString()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<1><279012><tt>";
-		$this->assertEquals(array('1', '279012'), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test with a ' ' in it.
-	 *
-	 * @todo is this valid input? Otherwise: should there be an exception?
-	 */
-	public function testExpandAclSpace()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<1><279 012><32>";
-		$this->assertEquals(array('1', '32'), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test empty input
-	 */
-	public function testExpandAclEmpty()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="";
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, no < at all
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclNoBrackets()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="According to documentation, that's invalid. "; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, just open <
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclJustOneBracket1()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<Another invalid string"; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, just close >
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclJustOneBracket2()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="Another invalid> string"; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, just close >
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclCloseOnly()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="Another> invalid> string>"; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, just open <
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclOpenOnly()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<Another< invalid string<"; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, open and close do not match
-	 *
-	 * @todo should there be an exception?
-	 */
-	public function testExpandAclNoMatching1()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<Another<> invalid <string>"; //should be invalid
-		$this->assertEquals(array(), $aclFormatter->expand($text));
-	}
-
-	/**
-	 * test invalid input, empty <>
-	 *
-	 * @todo should there be an exception? Or array(1, 3)
-	 * (This should be array(1,3) - mike)
-	 */
-	public function testExpandAclEmptyMatch()
-	{
-		$aclFormatter = new ACLFormatter();
-
-		$text="<1><><3>";
-		$this->assertEquals(array('1', '3'), $aclFormatter->expand($text));
+		$this->assertAcl($input, $assert);
 	}
 
 	/**
@@ -170,8 +111,14 @@ class ACLFormaterTest extends TestCase
 	{
 		$aclFormatter = new ACLFormatter();
 
-		$this->assertNull($aclFormatter->expand(null));
-		$this->assertNull($aclFormatter->expand());
+		$allow_people = $aclFormatter->expand();
+		$allow_groups = $aclFormatter->expand();
+
+		$this->assertEmpty($aclFormatter->expand(null));
+		$this->assertEmpty($aclFormatter->expand());
+
+		$recipients   = array_unique(array_merge($allow_people, $allow_groups));
+		$this->assertEmpty($recipients);
 	}
 
 	public function dataAclToString()