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/tests/ezFunctionsTest.php b/tests/ezFunctionsTest.php
index d91b3ac..6cd3eb7 100644
--- a/tests/ezFunctionsTest.php
+++ b/tests/ezFunctionsTest.php
@@ -176,13 +176,13 @@ 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()