diff --git a/framework/db/QueryBuilder.php b/framework/db/QueryBuilder.php
index 8e7d7e8..715c641 100644
--- a/framework/db/QueryBuilder.php
+++ b/framework/db/QueryBuilder.php
@@ -177,7 +177,8 @@ class QueryBuilder extends \yii\base\Object
 				if (!is_array($value) && isset($columnSchemas[$columns[$i]])) {
 					$value = $columnSchemas[$columns[$i]]->typecast($value);
 				}
-				$vs[] = is_string($value) ? $this->db->quoteValue($value) : $value;
+				$vs[] = is_string($value) ? $this->db->quoteValue($value)
+					: ( is_null($value) ? 'null' : $value );
 			}
 			$values[] = '(' . implode(', ', $vs) . ')';
 		}