diff --git a/wp-includes/general-template.php b/wp-includes/general-template.php
index e86963e6e..63e6d6524 100644
--- a/wp-includes/general-template.php
+++ b/wp-includes/general-template.php
@@ -2100,7 +2100,7 @@ function wp_admin_css( $file = 'wp-admin', $force_echo = false ) {
}
echo apply_filters( 'wp_admin_css', "\n", $file );
- if ( is_rtl() )
+ if ( function_exists( 'is_rtl' ) && is_rtl() )
echo apply_filters( 'wp_admin_css', "\n", "$file-rtl" );
}