diff --git a/core/DB/Dialect.class.php b/core/DB/Dialect.class.php
index 5c6b9e4641..1323e24b9f 100644
--- a/core/DB/Dialect.class.php
+++ b/core/DB/Dialect.class.php
@@ -100,17 +100,20 @@ public function toValueString($expression)
 		
 		private function toNeededString($expression, $method)
 		{
+			if (null === $expression)
+				throw new WrongArgumentException(
+					'not null expression expected'
+				);
+			
 			$string = null;
 			
-			if (null !== $expression) {
-				if ($expression instanceof DialectString) {
-					if ($expression instanceof Query)
-						$string .= '('.$expression->toDialectString($this).')';
-					else
-						$string .= $expression->toDialectString($this);
-				} else {
-					$string .= $this->$method($expression);
-				}
+			if ($expression instanceof DialectString) {
+				if ($expression instanceof Query)
+					$string .= '('.$expression->toDialectString($this).')';
+				else
+					$string .= $expression->toDialectString($this);
+			} else {
+				$string .= $this->$method($expression);
 			}
 			
 			return $string;
diff --git a/doc/ChangeLog b/doc/ChangeLog
index 5407f828ee..c19bd04dbc 100644
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@ -1,3 +1,8 @@
+2012-01-11	Evgeny V. Kokovikhin
+
+	* core/DB/Dialect.class.php: throw exception for null values. Thanks to 
+	Nikita V. Konstantinov.
+
 2011-11-21      Alexey S. Denisov, Evgeny V. Kokovikhin
 	
 	* meta/types/ObjectType.class.php, test/misc/DAOTest.class.php: changed logic
diff --git a/test/core/LogicTest.class.php b/test/core/LogicTest.class.php
index bfaa3834e7..83b20e5c4c 100644
--- a/test/core/LogicTest.class.php
+++ b/test/core/LogicTest.class.php
@@ -210,6 +210,14 @@ public function testBaseSqlGeneration()
 				'(- a)',
 				Expression::minus('a')->toDialectString($dialect)
 			);
+			
+			try {
+				Expression::eq('id', null)->toDialectString($dialect);
+				
+				$this->fail();
+			} catch (WrongArgumentException $e) {
+				//it's Ok
+			}
 		}
 		
 		public function testPgGeneration()