Index: src/wp-includes/option.php
===================================================================
--- src/wp-includes/option.php	(revision 33008)
+++ src/wp-includes/option.php	(working copy)
@@ -326,7 +326,7 @@
 
 	if ( ! defined( 'WP_INSTALLING' ) ) {
 		$alloptions = wp_load_alloptions();
-		if ( isset( $alloptions[$option] ) ) {
+		if ( array_key_exists( $option, $alloptions ) ) {
 			$alloptions[ $option ] = $serialized_value;
 			wp_cache_set( 'alloptions', $alloptions, 'options' );
 		} else {