diff --git a/lib/Constants.php b/lib/Constants.php
index 0648011..c24c1f3 100644
--- a/lib/Constants.php
+++ b/lib/Constants.php
@@ -39,8 +39,8 @@
     \define('_BETWEEN', 'BETWEEN');
     \define('_notBETWEEN', 'NOT BETWEEN');
 
-    \define('_isNULL', 'IS NULL');
-    \define('_notNULL', 'IS NOT NULL');
+    \define('_isNULL', 'IS');
+    \define('_notNULL', 'IS NOT');
     \define('_BOOLEAN_OPERATORS', [
         '<', '>', '=', '!=', '>=', '<=', '<>',
         'IN', 'LIKE', 'NOT LIKE', 'BETWEEN', 'NOT BETWEEN', 'IS', 'IS NOT'
diff --git a/lib/ezFunctions.php b/lib/ezFunctions.php
index 5839664..bdc29c7 100644
--- a/lib/ezFunctions.php
+++ b/lib/ezFunctions.php
@@ -353,7 +353,7 @@ function gte($x, $y, $and = null, ...$args)
      *
      * @return array
      */
-    function isNull($x, $y = 'null', $and = null, ...$args)
+    function isNull($x, $y = 'NULL', $and = null, ...$args)
     {
         $expression = array();
         \array_push($expression, $x, \_isNULL, $y, $and, ...$args);
@@ -370,7 +370,7 @@ function isNull($x, $y = 'null', $and = null, ...$args)
      *
      * @return array
      */
-    function isNotNull($x, $y = 'null', $and = null, ...$args)
+    function isNotNull($x, $y = 'NULL', $and = null, ...$args)
     {
         $expression = array();
         \array_push($expression, $x, \_notNULL, $y, $and, ...$args);
diff --git a/tests/ezFunctionsTest.php b/tests/ezFunctionsTest.php
index cf685cc..bee8b52 100644
--- a/tests/ezFunctionsTest.php
+++ b/tests/ezFunctionsTest.php
@@ -174,13 +174,13 @@ public function testGte()
     public function testIsNull()
     {
         $this->assertIsArray(isNull('field'));
-        $this->assertArraySubset([2 => 'null'], isNull('field'));
+        $this->assertArraySubset([2 => 'NULL'], isNull('field'));
     }
 
     public function testIsNotNull()
     {
         $this->assertIsArray(isNotNull('field'));
-        $this->assertArraySubset([2 => 'null'], isNotNull('field'));
+        $this->assertArraySubset([2 => 'NULL'], isNotNull('field'));
     }
 
     public function testLike()