diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index aa507bb004..81409e9382 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -8221,21 +8221,50 @@ function recurse_dirsize( $directory, $exclude = null, $max_execution_time = nul
  * Removes the current directory and all parent directories from the `dirsize_cache` transient.
  *
  * @since 5.6.0
+ * @since 5.9.0 Added input validation with a notice for invalid input.
  *
  * @param string $path Full path of a directory or file.
  */
 function clean_dirsize_cache( $path ) {
+	if ( ! is_string( $path ) || empty( $path ) ) {
+		trigger_error(
+			sprintf(
+				/* translators: 1: Function name, 2: A variable type, like "boolean" or "integer". */
+				__( '%1$s only accepts a non-empty path string, received %2$s.' ),
+				'<code>clean_dirsize_cache()</code>',
+				'<code>' . gettype( $path ) . '</code>'
+			)
+		);
+		return;
+	}
+
 	$directory_cache = get_transient( 'dirsize_cache' );
 
 	if ( empty( $directory_cache ) ) {
 		return;
 	}
 
-	$path = untrailingslashit( $path );
+	if (
+		strpos( $path, '/' ) === false &&
+		strpos( $path, '\\' ) === false
+	) {
+		unset( $directory_cache[ $path ] );
+		set_transient( 'dirsize_cache', $directory_cache );
+		return;
+	}
+
+	$last_path = null;
+	$path      = untrailingslashit( $path );
 	unset( $directory_cache[ $path ] );
 
-	while ( DIRECTORY_SEPARATOR !== $path && '.' !== $path && '..' !== $path ) {
-		$path = dirname( $path );
+	while (
+		$last_path !== $path &&
+		DIRECTORY_SEPARATOR !== $path &&
+		'.' !== $path &&
+		'..' !== $path
+	) {
+		$last_path = $path;
+		$path      = dirname( $path );
 		unset( $directory_cache[ $path ] );
 	}
 
diff --git a/tests/phpunit/tests/functions/cleanDirsizeCache.php b/tests/phpunit/tests/functions/cleanDirsizeCache.php
new file mode 100644
index 0000000000..6ae7d6d2dc
--- /dev/null
+++ b/tests/phpunit/tests/functions/cleanDirsizeCache.php
@@ -0,0 +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