diff --git includes/common.inc includes/common.inc
index e749880..6911787 100644
--- includes/common.inc
+++ includes/common.inc
@@ -162,7 +162,7 @@ define('DRUPAL_CACHE_GLOBAL', 0x0008);
 function drupal_add_region_content($region = NULL, $data = NULL) {
   static $content = array();
 
-  if (!is_null($region) && !is_null($data)) {
+  if (isset($region) && isset($data)) {
     $content[$region][] = $data;
   }
   return $content;
@@ -230,7 +230,7 @@ function drupal_get_profile() {
 function drupal_set_breadcrumb($breadcrumb = NULL) {
   $stored_breadcrumb = &drupal_static(__FUNCTION__);
 
-  if (!is_null($breadcrumb)) {
+  if (isset($breadcrumb)) {
     $stored_breadcrumb = $breadcrumb;
   }
   return $stored_breadcrumb;
@@ -6090,7 +6090,7 @@ function drupal_get_schema_unprocessed($module, $table = NULL) {
   module_load_install($module);
   $schema = module_invoke($module, 'schema');
 
-  if (!is_null($table) && isset($schema[$table])) {
+  if (isset($table) && isset($schema[$table])) {
     return $schema[$table];
   }
   elseif (!empty($schema)) {
@@ -6223,7 +6223,7 @@ function drupal_write_record($table, &$record, $primary_keys = array()) {
     // MySQL PDO silently casts e.g. FALSE and '' to 0 when inserting the value
     // into an integer column, but PostgreSQL PDO does not. Also type cast NULL
     // when the column does not allow this.
-    if (!is_null($object->$field) || !empty($info['not null'])) {
+    if (isset($object->$field) || !empty($info['not null'])) {
       if ($info['type'] == 'int' || $info['type'] == 'serial') {
         $fields[$field] = (int) $fields[$field];
       }
@@ -6793,10 +6793,10 @@ function entity_create_stub_entity($entity_type, $ids) {
   $entity = new stdClass();
   $info = entity_get_info($entity_type);
   $entity->{$info['entity keys']['id']} = $ids[0];
-  if (!empty($info['entity keys']['revision']) && !is_null($ids[1])) {
+  if (!empty($info['entity keys']['revision']) && isset($ids[1])) {
     $entity->{$info['entity keys']['revision']} = $ids[1];
   }
-  if (!empty($info['entity keys']['bundle']) && !is_null($ids[2])) {
+  if (!empty($info['entity keys']['bundle']) && isset($ids[2])) {
     $entity->{$info['entity keys']['bundle']} = $ids[2];
   }
   return $entity;
diff --git includes/file.inc includes/file.inc
index 6a18aee..1037ce9 100644
--- includes/file.inc
+++ includes/file.inc
@@ -1296,7 +1296,7 @@ function file_space_used($uid = NULL, $status = FILE_STATUS_PERMANENT) {
   $query = db_select('file_managed', 'f');
   $query->condition('f.status', $status);
   $query->addExpression('SUM(f.filesize)', 'filesize');
-  if (!is_null($uid)) {
+  if (isset($uid)) {
     $query->condition('f.uid', $uid);
   }
   return $query->execute()->fetchField();
diff --git modules/field/field.crud.inc modules/field/field.crud.inc
index e2ec08b..13b26fc 100644
--- modules/field/field.crud.inc
+++ modules/field/field.crud.inc
@@ -762,7 +762,7 @@ function _field_write_instance($instance, $update = FALSE) {
   // If no weight specified, make sure the field sinks at the bottom.
   if (!isset($instance['widget']['weight'])) {
     $max_weight = field_info_max_weight($instance['entity_type'], $instance['bundle'], 'form');
-    $instance['widget']['weight'] = !is_null($max_weight) ? $max_weight + 1 : 0;
+    $instance['widget']['weight'] = isset($max_weight) ? $max_weight + 1 : 0;
   }
   // Check widget module.
   $widget_type = field_info_widget_types($instance['widget']['type']);
@@ -788,7 +788,7 @@ function _field_write_instance($instance, $update = FALSE) {
     // If no weight specified, make sure the field sinks at the bottom.
     if (!isset($display['weight'])) {
       $max_weight = field_info_max_weight($instance['entity_type'], $instance['bundle'], $view_mode);
-      $display['weight'] = !is_null($max_weight) ? $max_weight + 1 : 0;
+      $display['weight'] = isset($max_weight) ? $max_weight + 1 : 0;
     }
     $instance['display'][$view_mode] = $display;
   }
diff --git modules/system/system.admin.inc modules/system/system.admin.inc
index 94ae7e7..2b717cb 100644
--- modules/system/system.admin.inc
+++ modules/system/system.admin.inc
@@ -639,7 +639,7 @@ function system_theme_settings($form, &$form_state, $key = '') {
     }
 
     // Restore the original current theme.
-    if (!is_null($default_theme)) {
+    if (isset($default_theme)) {
       $GLOBALS['theme_key'] = $default_theme;
     }
     else {