diff --git a/wp-includes/general-template.php b/wp-includes/general-template.php
index e86963e6ee..63e6d6524a 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', "<link rel='stylesheet' href='" . esc_url( wp_admin_css_uri( $file ) ) . "' type='text/css' />\n", $file );
-	if ( is_rtl() )
+	if ( function_exists( 'is_rtl' ) && is_rtl() )
 		echo apply_filters( 'wp_admin_css', "<link rel='stylesheet' href='" . esc_url( wp_admin_css_uri( "$file-rtl" ) ) . "' type='text/css' />\n", "$file-rtl" );
 }