diff --git a/tests/phpunit/data/functions/dummy.txt b/tests/phpunit/data/functions/dummy.txt
new file mode 100644
index 0000000000..7614ea709c
--- /dev/null
+++ b/tests/phpunit/data/functions/dummy.txt
@@ -0,0 +1 @@
+This is a dummy text file which is only used by the `Tests_Functions_CleanDirsizeCache::test_recurse_dirsize_without_transient()` test.
diff --git a/tests/phpunit/tests/functions/cleanDirsizeCache.php b/tests/phpunit/tests/functions/cleanDirsizeCache.php
index d600bdca54..27aacda62b 100644
--- a/tests/phpunit/tests/functions/cleanDirsizeCache.php
+++ b/tests/phpunit/tests/functions/cleanDirsizeCache.php
@@ -1 +1,140 @@
-<?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
+<?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_TESTDATA . '/functions' );
+
+		$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_TESTDATA . '/functions' );
+
+		$this->assertGreaterThan( 10, $size );
+	}
+}
diff --git a/tests/phpunit/tests/functions/fixtures/dummy.txt b/tests/phpunit/tests/functions/fixtures/dummy.txt
deleted file mode 100644
index 7cc0f577ee..0000000000
--- a/tests/phpunit/tests/functions/fixtures/dummy.txt
+++ /dev/null
@@ -1 +0,0 @@
-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