Index: wp-includes/cache.php
===================================================================
--- wp-includes/cache.php	(revision 18557)
+++ wp-includes/cache.php	(working copy)
@@ -24,7 +24,8 @@
 function wp_cache_add($key, $data, $flag = '', $expire = 0) {
 	global $wp_object_cache;
 
-	return $wp_object_cache->add($key, $data, $flag, $expire);
+    if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'add' ) )
+        return $wp_object_cache->add($key, $data, $flag, $expire);
 }
 
 /**
@@ -57,7 +58,8 @@
 function wp_cache_delete($id, $flag = '') {
 	global $wp_object_cache;
 
-	return $wp_object_cache->delete($id, $flag);
+	if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'delete' ) )
+        return $wp_object_cache->delete($id, $flag);
 }
 
 /**
@@ -72,7 +74,8 @@
 function wp_cache_flush() {
 	global $wp_object_cache;
 
-	return $wp_object_cache->flush();
+	if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'flush' ) )
+        return $wp_object_cache->flush();
 }
 
 /**
@@ -90,7 +93,8 @@
 function wp_cache_get($id, $flag = '') {
 	global $wp_object_cache;
 
-	return $wp_object_cache->get($id, $flag);
+	if ( !empty( $wp_object_cache ) )
+        return $wp_object_cache->get($id, $flag);
 }
 
 /**
@@ -119,7 +123,8 @@
 function wp_cache_replace($key, $data, $flag = '', $expire = 0) {
 	global $wp_object_cache;
 
-	return $wp_object_cache->replace($key, $data, $flag, $expire);
+	if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'replace' ) )
+        return $wp_object_cache->replace($key, $data, $flag, $expire);
 }
 
 /**
@@ -138,7 +143,8 @@
 function wp_cache_set($key, $data, $flag = '', $expire = 0) {
 	global $wp_object_cache;
 
-	return $wp_object_cache->set($key, $data, $flag, $expire);
+	if ( !empty( $wp_object_cache ) )
+        return $wp_object_cache->set($key, $data, $flag, $expire);
 }
 
 /**
@@ -151,7 +157,8 @@
 function wp_cache_add_global_groups( $groups ) {
 	global $wp_object_cache;
 
-	return $wp_object_cache->add_global_groups($groups);
+	if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'add_global_groups' ) )
+        return $wp_object_cache->add_global_groups($groups);
 }
 
 /**
@@ -175,7 +182,8 @@
 function wp_cache_reset() {
 	global $wp_object_cache;
 
-	return $wp_object_cache->reset();
+	if ( !empty( $wp_object_cache ) && method_exists( $wp_object_cache , 'reset' ) )
+        return $wp_object_cache->reset();
 }
 
 /**