diff --git a/object-cache.php b/object-cache.php
index 67fedb4066753640a73b5c6c0521103773a6c5a6..0a66dd71975c069849e363c556161af3e7a45a9c 100644
--- a/object-cache.php
+++ b/object-cache.php
@@ -370,8 +370,10 @@ class WP_Object_Cache {
 		}
 
 		// Assign global and blog prefixes for use with keys
-		$this->global_prefix = ( is_multisite() || defined( 'CUSTOM_USER_TABLE' ) && defined( 'CUSTOM_USER_META_TABLE' ) ) ? '' : $table_prefix;
-		$this->blog_prefix   = ( is_multisite() ? $blog_id : $table_prefix ) . ':';
+		if ( function_exists( 'is_multisite' ) ) {
+			$this->global_prefix = ( is_multisite() || defined( 'CUSTOM_USER_TABLE' ) && defined( 'CUSTOM_USER_META_TABLE' ) ) ? '' : $table_prefix;
+			$this->blog_prefix   = ( is_multisite() ? $blog_id : $table_prefix ) . ':';
+		}
 	}
 
 	/**
@@ -845,7 +847,7 @@ class WP_Object_Cache {
 	 * @return bool
 	 */
 	public function switch_to_blog( $_blog_id ) {
-		if ( ! is_multisite() ) {
+		if ( ! function_exists( 'is_multisite' ) || ! is_multisite() ) {
 			return false;
 		}