diff --git a/search_api.views.inc b/search_api.views.inc
index 679dc842..f47fc123 100644
--- a/search_api.views.inc
+++ b/search_api.views.inc
@@ -220,7 +220,7 @@ function _search_api_views_get_handlers(FieldInterface $field) {
 function _search_api_views_handler_mapping() {
   $mapping = &drupal_static(__FUNCTION__);
 
-  if (!isset($mapping)) {
+  if ($mapping === NULL) {
     $mapping = [
       'boolean' => [
         'argument' => [
@@ -825,7 +825,7 @@ function _search_api_views_get_field_handler_for_property(DataDefinitionInterfac
 function _search_api_views_get_field_handler_mapping() {
   $mappings = &drupal_static(__FUNCTION__);
 
-  if (!isset($mappings)) {
+  if ($mappings === NULL) {
     // First create a plain mapping and pass it to the alter hook.
     $plain_mapping = [];
 
diff --git a/src/Plugin/search_api/data_type/DateDataType.php b/src/Plugin/search_api/data_type/DateDataType.php
index f7734523..bd72ff2c 100644
--- a/src/Plugin/search_api/data_type/DateDataType.php
+++ b/src/Plugin/search_api/data_type/DateDataType.php
@@ -29,7 +29,7 @@ public function getValue($value) {
       return (int) $value;
     }
 
-    $timezone = new \DateTimezone(DateTimeItemInterface::STORAGE_TIMEZONE);
+    $timezone = new \DateTimeZone(DateTimeItemInterface::STORAGE_TIMEZONE);
     $date = new DateTimePlus($value, $timezone);
     // Add in time component if this is a date-only field.
     if (strpos($value, ':') === FALSE) {
diff --git a/src/Plugin/search_api/datasource/ContentEntity.php b/src/Plugin/search_api/datasource/ContentEntity.php
index 6130fc6c..6224ce31 100644
--- a/src/Plugin/search_api/datasource/ContentEntity.php
+++ b/src/Plugin/search_api/datasource/ContentEntity.php
@@ -951,7 +951,8 @@ protected function getPropertyPathDependencies($property_path, array $properties
       }
     }
 
-    if (isset($nested_path) && $property instanceof ComplexDataDefinitionInterface) {
+    if ($nested_path !== NULL
+        && $property instanceof ComplexDataDefinitionInterface) {
       $nested = $this->getFieldsHelper()->getNestedProperties($property);
       $nested_dependencies = $this->getPropertyPathDependencies($nested_path, $nested);
       foreach ($nested_dependencies as $type => $names) {
diff --git a/src/Utility/CommandHelper.php b/src/Utility/CommandHelper.php
index ad6fc207..86b68f14 100644
--- a/src/Utility/CommandHelper.php
+++ b/src/Utility/CommandHelper.php
@@ -295,7 +295,7 @@ public function indexItemsToIndexCommand(array $indexIds = NULL, $limit = NULL,
       }
 
       // Get the number items to index.
-      if (!isset($current_limit) || !is_int($current_limit += 0) || $current_limit <= 0) {
+      if (!is_int($current_limit += 0) || $current_limit <= 0) {
         $current_limit = $remaining;
       }
 
diff --git a/src/Utility/FieldsHelper.php b/src/Utility/FieldsHelper.php
index cc2c8096..c9abe8b3 100644
--- a/src/Utility/FieldsHelper.php
+++ b/src/Utility/FieldsHelper.php
@@ -352,7 +352,7 @@ public function retrieveNestedProperty(array $properties, $propertyPath) {
     }
 
     $property = $this->getInnerProperty($properties[$key]);
-    if (!isset($nestedPath)) {
+    if ($nestedPath === NULL) {
       return $property;
     }
 
diff --git a/tests/src/Unit/Processor/TestFieldsProcessorPlugin.php b/tests/src/Unit/Processor/TestFieldsProcessorPlugin.php
index 33a1a531..14414f97 100644
--- a/tests/src/Unit/Processor/TestFieldsProcessorPlugin.php
+++ b/tests/src/Unit/Processor/TestFieldsProcessorPlugin.php
@@ -124,7 +124,7 @@ protected function process(&$value) {
       $this->methodOverrides[__FUNCTION__]($value);
       return;
     }
-    if (isset($value)) {
+    if ($value !== NULL) {
       $value = "*$value";
     }
     else {