=== modified file 'includes/database/query.inc'
--- includes/database/query.inc	2010-05-15 07:04:21 +0000
+++ includes/database/query.inc	2010-05-31 00:17:22 +0000
@@ -1212,7 +1212,15 @@ class DatabaseCondition implements Query
 
   public function condition($field, $value = NULL, $operator = NULL) {
     if (!isset($operator)) {
-      $operator = is_array($value) ? 'IN' : '=';
+      if (is_array($value)) {
+        $operator = 'IN';
+      }
+      elseif (!isset($value)) {
+        $operator = 'IS NULL';
+      }
+      else {
+        $operator = '=';
+      }
     }
     $this->conditions[] = array(
       'field' => $field,
@@ -1237,11 +1245,11 @@ class DatabaseCondition implements Query
   }
 
   public function isNull($field) {
-    return $this->condition($field, NULL, 'IS NULL');
+    return $this->condition($field);
   }
 
   public function isNotNull($field) {
-    return $this->condition($field, NULL, 'IS NOT NULL');
+    return $this->condition($field, NULL, 'NOT NULL');
   }
 
   public function &conditions() {