Index: wp-includes/wp-db.php
===================================================================
--- wp-includes/wp-db.php	(revision 28567)
+++ wp-includes/wp-db.php	(working copy)
@@ -1785,8 +1785,10 @@
 			$wheres[] = "`$field` = {$form}";
 		}
 
-		$sql = "UPDATE `$table` SET " . implode( ', ', $bits ) . ' WHERE ' . implode( ' AND ', $wheres );
-		return $this->query( $this->prepare( $sql, array_merge( array_values( $data ), array_values( $where ) ) ) );
+		$wheres = empty ( $where ) ? '1 = 1' : implode( ' AND ', $wheres );
+
+		$sql = "UPDATE `$table` SET " . implode( ', ', $bits ) . ' WHERE ' . $wheres;
+		return $this->query( $this->prepare( $sql, array_merge( array_values( $data ), $where ) ) );
 	}
 
 	/**