diff --git includes/database/mysql/schema.inc includes/database/mysql/schema.inc
index 3a68155..65d55fe 100644
--- includes/database/mysql/schema.inc
+++ includes/database/mysql/schema.inc
@@ -156,7 +156,7 @@ class DatabaseSchema_mysql extends DatabaseSchema {
       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 @@ class DatabaseSchema_mysql extends DatabaseSchema {
       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 includes/database/pgsql/schema.inc includes/database/pgsql/schema.inc
index ca923a5..4300d3f 100644
--- includes/database/pgsql/schema.inc
+++ includes/database/pgsql/schema.inc
@@ -353,7 +353,7 @@ class DatabaseSchema_pgsql extends DatabaseSchema {
       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 includes/database/select.inc includes/database/select.inc
index 3430001..451eac1 100644
--- includes/database/select.inc
+++ includes/database/select.inc
@@ -675,7 +675,7 @@ class SelectQueryExtender implements SelectQueryInterface {
 
   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 includes/database/sqlite/database.inc includes/database/sqlite/database.inc
index e6f2408..f6566b5 100644
--- includes/database/sqlite/database.inc
+++ includes/database/sqlite/database.inc
@@ -82,7 +82,7 @@ class DatabaseConnection_sqlite extends DatabaseConnection {
   public function sqlFunctionGreatest() {
     $args = func_get_args();
     foreach ($args as $k => $v) {
-      if (is_null($v)) {
+      if (!isset($v)) {
         unset($args);
       }
     }
diff --git includes/database/sqlite/query.inc includes/database/sqlite/query.inc
index f519eeb..87b82d1 100644
--- includes/database/sqlite/query.inc
+++ includes/database/sqlite/query.inc
@@ -110,7 +110,7 @@ class UpdateQuery_sqlite extends UpdateQuery {
         $condition->condition($field, $data['expression'], '<>');
         $condition->isNull($field);
       }
-      elseif (is_null($data)) {
+      elseif (!isset($data)) {
         // The field will be set to NULL.
         $condition->isNull($field);
       }