diff --git a/includes/database/mysql/schema.inc b/includes/database/mysql/schema.inc
index 1712ca2a459cea0ed0285baf53555d179ae1bcaf..cba41eb5f7c085e58d38bbb18c0b0ca7815f44c1 100644
--- a/includes/database/mysql/schema.inc
+++ b/includes/database/mysql/schema.inc
@@ -156,7 +156,7 @@ protected function createFieldSql($name, $spec) {
       if (is_string($spec['default'])) {
         $spec['default'] = "'" . $spec['default'] . "'";
       }
-      elseif (is_null($spec['default'])) {
+      elseif (!isset($spec['default'])) {
         $spec['default'] = 'NULL';
       }
       $sql .= ' DEFAULT ' . $spec['default'];
@@ -351,7 +351,7 @@ public function fieldSetDefault($table, $field, $default) {
       throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot set default value of field %table.%field: field doesn't exist.", array('%table' => $table, '%field' => $field)));
     }
 
-    if (is_null($default)) {
+    if (!isset($default)) {
       $default = 'NULL';
     }
     else {
diff --git a/includes/database/pgsql/schema.inc b/includes/database/pgsql/schema.inc
index 4c9ce4638c767152f86e5b62a309240dcfa59cbb..cfdb312775a3d4207c5df018ef23313b4b1a2368 100644
--- a/includes/database/pgsql/schema.inc
+++ b/includes/database/pgsql/schema.inc
@@ -353,7 +353,7 @@ public function fieldSetDefault($table, $field, $default) {
       throw new DatabaseSchemaObjectDoesNotExistException(t("Cannot set default value of field %table.%field: field doesn't exist.", array('%table' => $table, '%field' => $field)));
     }
 
-    if (is_null($default)) {
+    if (!isset($default)) {
       $default = 'NULL';
     }
     else {
diff --git a/includes/database/select.inc b/includes/database/select.inc
index 72d3454b640ad59f4633651447bf3c40d1934f92..fd4bd7f87ce710578d88ca440ff55e0097031e85 100644
--- a/includes/database/select.inc
+++ b/includes/database/select.inc
@@ -682,7 +682,7 @@ public function isPrepared() {
 
   public function preExecute(SelectQueryInterface $query = NULL) {
     // If no query object is passed in, use $this.
-    if (is_null($query)) {
+    if (!isset($query)) {
       $query = $this;
     }
 
@@ -1147,7 +1147,7 @@ public function isPrepared() {
    */
   public function preExecute(SelectQueryInterface $query = NULL) {
     // If no query object is passed in, use $this.
-    if (is_null($query)) {
+    if (!isset($query)) {
       $query = $this;
     }
 
diff --git a/includes/database/sqlite/database.inc b/includes/database/sqlite/database.inc
index 4b7aaa6206f88a4f4ff28bfde494d890f829039e..476876d3ef4ebc541379d523db1a4e2c3b3c5f33 100644
--- a/includes/database/sqlite/database.inc
+++ b/includes/database/sqlite/database.inc
@@ -82,7 +82,7 @@ public function sqlFunctionIf($condition, $expr1, $expr2 = NULL) {
   public function sqlFunctionGreatest() {
     $args = func_get_args();
     foreach ($args as $k => $v) {
-      if (is_null($v)) {
+      if (!isset($v)) {
         unset($args);
       }
     }
diff --git a/includes/database/sqlite/query.inc b/includes/database/sqlite/query.inc
index c79da03289935b9c6192170930c6d89e74db8c86..ec2b57a2c05b90392e41a3be328791da4ecbff87 100644
--- a/includes/database/sqlite/query.inc
+++ b/includes/database/sqlite/query.inc
@@ -110,7 +110,7 @@ public function execute() {
         $condition->condition($field, $data['expression'], '<>');
         $condition->isNull($field);
       }
-      elseif (is_null($data)) {
+      elseif (!isset($data)) {
         // The field will be set to NULL.
         $condition->isNull($field);
       }