diff --git a/tests/phpunit/includes/class-wp-test-stream.php b/tests/phpunit/includes/class-wp-test-stream.php
index 11997507d0..17d930b586 100644
--- a/tests/phpunit/includes/class-wp-test-stream.php
+++ b/tests/phpunit/includes/class-wp-test-stream.php
@@ -198,16 +198,27 @@ class WP_Test_Stream {
 	 * Creates a directory.
 	 *
 	 * @see streamWrapper::mkdir
+	 *
+	 * @param string $path    Directory which should be created.
+	 * @param int    $mode    The value passed to mkdir().
+	 * @param int    $options A bitwise mask of values, such as STREAM_MKDIR_RECURSIVE.
+	 * @return bool True on success, false on failure.
 	 */
 	public function mkdir( $path, $mode, $options ) {
 		$this->open( $path );
+
 		$plainfile = rtrim( $this->file, '/' );
 
-		if ( isset( WP_Test_Stream::$data[ $this->bucket ][ $file ] ) ) {
+		// Check if a file or directory with the same name already exists.
+		if ( isset( WP_Test_Stream::$data[ $this->bucket ][ $plainfile ] )
+			|| isset( WP_Test_Stream::$data[ $this->bucket ][ $plainfile . '/' ] )
+		) {
 			return false;
 		}
+
 		$dir_ref = & $this->get_directory_ref();
 		$dir_ref = 'DIRECTORY';
+
 		return true;
 	}