diff --git a/object-cache.php b/object-cache.php
index 622c1e9..dd2ff80 100644
--- a/object-cache.php
+++ b/object-cache.php
@@ -477,7 +477,7 @@ function flush() {
 
 		$this->rotate_site_keys( $flush_number );
 
-		if ( is_main_site() ) {
+		if ( function_exists( 'is_main_site' ) && is_main_site() ) {
 			$this->rotate_global_keys( $flush_number );
 		}