diff --git src/wp-includes/functions.php src/wp-includes/functions.php
index 58a098f916..cbd17d211d 100644
--- src/wp-includes/functions.php
+++ src/wp-includes/functions.php
@@ -5513,7 +5513,7 @@ function wp_guess_url() {
 				$directory = str_replace( ABSPATH, '', $script_filename_dir );
 				// Strip off the subdirectory, and any file/query params.
 				$path = preg_replace( '#/' . preg_quote( $directory, '#' ) . '/[^/]*$#i', '', $_SERVER['REQUEST_URI'] );
-			} elseif ( false !== strpos( $abspath_fix, $script_filename_dir ) ) {
+			} elseif ( '' !== $script_filename_dir && false !== strpos( $abspath_fix, $script_filename_dir ) ) {
 				// Request is hitting a file above ABSPATH.
 				$subdirectory = substr( $abspath_fix, strpos( $abspath_fix, $script_filename_dir ) + strlen( $script_filename_dir ) );
 				// Strip off any file/query params from the path, appending the subdirectory to the installation.