diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 81409e9382..86a21a8e78 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -8120,8 +8120,9 @@ function get_dirsize( $directory, $max_execution_time = null ) {
  * @param string       $directory          Full path of a directory.
  * @param string|array $exclude            Optional. Full path of a subdirectory to exclude from the total,
  *                                         or array of paths. Expected without trailing slash(es).
- * @param int          $max_execution_time Maximum time to run before giving up. In seconds. The timeout is global
- *                                         and is measured from the moment WordPress started to load.
+ * @param int          $max_execution_time Optional. Maximum time to run before giving up. In seconds.
+ *                                         The timeout is global and is measured from the moment
+ *                                         WordPress started to load.
  * @param array        $directory_cache    Optional. Array of cached directory paths.
  *
  * @return int|false|null Size in bytes if a valid directory. False if not. Null if timeout.
@@ -8194,7 +8195,9 @@ function recurse_dirsize( $directory, $exclude = null, $max_execution_time = nul
 						}
 					}
 
-					if ( $max_execution_time > 0 && microtime( true ) - WP_START_TIMESTAMP > $max_execution_time ) {
+					if ( $max_execution_time > 0 &&
+						( microtime( true ) - WP_START_TIMESTAMP ) > $max_execution_time
+					) {
 						// Time exceeded. Give up instead of risking a fatal timeout.
 						$size = null;
 						break;
@@ -8205,6 +8208,10 @@ function recurse_dirsize( $directory, $exclude = null, $max_execution_time = nul
 		}
 	}
 
+	if ( ! is_array( $directory_cache ) ) {
+		$directory_cache = array();
+	}
+
 	$directory_cache[ $directory ] = $size;
 
 	// Only write the transient on the top level call and not on recursive calls.
diff --git a/tests/phpunit/tests/functions/cleanDirsizeCache.php b/tests/phpunit/tests/functions/cleanDirsizeCache.php
index 6ae7d6d2dc..d600bdca54 100644
--- a/tests/phpunit/tests/functions/cleanDirsizeCache.php
+++ b/tests/phpunit/tests/functions/cleanDirsizeCache.php
@@ -1 +1 @@
-<?php

/**
 * Tests specific to the directory size caching.
 *
 * @covers ::clean_dirsize_cache
 * @group functions.php
 */
class Tests_Functions_CleanDirsizeCache extends WP_UnitTestCase {

	/**
	 * Test the handling of invalid data passed as the $path parameter.
	 *
	 * @ticket 52241
	 *
	 * @dataProvider data_clean_dirsize_cache_with_invalid_inputs
	 *
	 * @param mixed  $path             Path input to use in the test.
	 * @param string $expected_message Expected notice message.
	 */
	public function test_clean_dirsize_cache_with_invalid_inputs( $path, $expected_message ) {
		$this->expectNotice();
		$this->expectNoticeMessage( $expected_message );

		clean_dirsize_cache( $path );
	}

	/**
	 * Data provider.
	 *
	 * @return array
	 */
	public function data_clean_dirsize_cache_with_invalid_inputs() {
		return array(
			'null'         => array(
				'path'             => null,
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>NULL</code>.',
			),
			'bool false'   => array(
				'path'             => false,
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>boolean</code>.',
			),
			'empty string' => array(
				'path'             => '',
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>string</code>.',
			),
			'array'        => array(
				'path'             => array( '.', './second/path/' ),
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>array</code>.',
			),
		);
	}

	/**
	 * Test the handling of a non-path text string passed as the $path parameter.
	 *
	 * @ticket 52241
	 *
	 * @dataProvider data_clean_dirsize_cache_with_non_path_string
	 *
	 * @param string $path           Path input to use in the test.
	 * @param int    $expected_count Expected number of paths in the cache after cleaning.
	 */
	public function test_clean_dirsize_cache_with_non_path_string( $path, $expected_count ) {
		// Set the dirsize cache to our mock.
		set_transient( 'dirsize_cache', $this->mock_dirsize_cache_with_non_path_string() );

		clean_dirsize_cache( $path );

		$cache = get_transient( 'dirsize_cache' );
		$this->assertIsArray( $cache );
		$this->assertCount( $expected_count, $cache );
	}

	/**
	 * Data provider.
	 *
	 * @return array
	 */
	public function data_clean_dirsize_cache_with_non_path_string() {
		return array(
			'single dot'                        => array(
				'path'           => '.',
				'expected_count' => 1,
			),
			'non-path'                          => array(
				'path'           => 'string',
				'expected_count' => 1,
			),
			'non-existant string, but non-path' => array(
				'path'           => 'doesnotexist',
				'expected_count' => 2,
			),
		);
	}

	private function mock_dirsize_cache_with_non_path_string() {
		return array(
			'.'      => array( 'size' => 50 ),
			'string' => array( 'size' => 42 ),
		);
	}
}
\ No newline at end of file
+<?php

/**
 * Tests specific to the directory size caching.
 *
 * @group functions.php
 */
class Tests_Functions_CleanDirsizeCache extends WP_UnitTestCase {

	/**
	 * Test the handling of invalid data passed as the $path parameter.
	 *
	 * @ticket 52241
	 *
	 * @covers ::clean_dirsize_cache
	 *
	 * @dataProvider data_clean_dirsize_cache_with_invalid_inputs
	 *
	 * @param mixed  $path             Path input to use in the test.
	 * @param string $expected_message Expected notice message.
	 */
	public function test_clean_dirsize_cache_with_invalid_inputs( $path, $expected_message ) {
		$this->expectNotice();
		$this->expectNoticeMessage( $expected_message );

		clean_dirsize_cache( $path );
	}

	/**
	 * Data provider.
	 *
	 * @return array
	 */
	public function data_clean_dirsize_cache_with_invalid_inputs() {
		return array(
			'null'         => array(
				'path'             => null,
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>NULL</code>.',
			),
			'bool false'   => array(
				'path'             => false,
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>boolean</code>.',
			),
			'empty string' => array(
				'path'             => '',
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>string</code>.',
			),
			'array'        => array(
				'path'             => array( '.', './second/path/' ),
				'expected_message' => '<code>clean_dirsize_cache()</code> only accepts a non-empty path string, received <code>array</code>.',
			),
		);
	}

	/**
	 * Test the handling of a non-path text string passed as the $path parameter.
	 *
	 * @ticket 52241
	 *
	 * @covers ::clean_dirsize_cache
	 *
	 * @dataProvider data_clean_dirsize_cache_with_non_path_string
	 *
	 * @param string $path           Path input to use in the test.
	 * @param int    $expected_count Expected number of paths in the cache after cleaning.
	 */
	public function test_clean_dirsize_cache_with_non_path_string( $path, $expected_count ) {
		// Set the dirsize cache to our mock.
		set_transient( 'dirsize_cache', $this->mock_dirsize_cache_with_non_path_string() );

		clean_dirsize_cache( $path );

		$cache = get_transient( 'dirsize_cache' );
		$this->assertIsArray( $cache );
		$this->assertCount( $expected_count, $cache );
	}

	/**
	 * Data provider.
	 *
	 * @return array
	 */
	public function data_clean_dirsize_cache_with_non_path_string() {
		return array(
			'single dot'                        => array(
				'path'           => '.',
				'expected_count' => 1,
			),
			'non-path'                          => array(
				'path'           => 'string',
				'expected_count' => 1,
			),
			'non-existant string, but non-path' => array(
				'path'           => 'doesnotexist',
				'expected_count' => 2,
			),
		);
	}

	private function mock_dirsize_cache_with_non_path_string() {
		return array(
			'.'      => array( 'size' => 50 ),
			'string' => array( 'size' => 42 ),
		);
	}

	/**
	 * Test the behaviour of the function when the transient doesn't exist.
	 *
	 * @ticket 52241
	 * @ticket 53635
	 *
	 * @covers ::recurse_dirsize
	 */
	public function test_recurse_dirsize_without_transient() {
		delete_transient( 'dirsize_cache' );

		$size = recurse_dirsize( __DIR__ . '/fixtures' );

		$this->assertGreaterThan( 10, $size );
	}

	/**
	 * Test the behaviour of the function when the transient does exist, but is not an array.
	 *
	 * In particular, this tests that no PHP TypeErrors are being thrown.
	 *
	 * @ticket 52241
	 * @ticket 53635
	 *
	 * @covers ::recurse_dirsize
	 */
	public function test_recurse_dirsize_with_invalid_transient() {
		set_transient( 'dirsize_cache', 'this is not a valid transient for dirsize cache' );

		$size = recurse_dirsize( __DIR__ . '/fixtures' );

		$this->assertGreaterThan( 10, $size );
	}
}
\ No newline at end of file
diff --git a/tests/phpunit/tests/functions/fixtures/dummy.txt b/tests/phpunit/tests/functions/fixtures/dummy.txt
new file mode 100644
index 0000000000..7cc0f577ee
--- /dev/null
+++ b/tests/phpunit/tests/functions/fixtures/dummy.txt
@@ -0,0 +1 @@
+This is a dummy text file which is only used by the `Tests_Multisite_CleanDirsizeCache::test_recurse_dirsize_without_transient()` test.
\ No newline at end of file