diff --git a/sources/application/search/criterionconversion/criteriontooql.class.inc.php b/sources/application/search/criterionconversion/criteriontooql.class.inc.php
index a565f5d3d6..cc8b69fd28 100644
--- a/sources/application/search/criterionconversion/criteriontooql.class.inc.php
+++ b/sources/application/search/criterionconversion/criteriontooql.class.inc.php
@@ -25,6 +25,7 @@ namespace Combodo\iTop\Application\Search\CriterionConversion;
 
 use AttributeDateTime;
 use AttributeDefinition;
+use AttributeExternalKey;
 use Combodo\iTop\Application\Search\AjaxSearchException;
 use Combodo\iTop\Application\Search\CriterionConversionAbstract;
 use Combodo\iTop\Application\Search\SearchForm;
@@ -149,6 +150,8 @@ class CriterionToOQL extends CriterionConversionAbstract
 			return "1";
 		}
 
+		$bFilterOnUndefined = false;
+		$sFilterOnUndefined = '';
 		try
 		{
 			$aAttributeDefs = \MetaModel::ListAttributeDefs($sClass);
@@ -182,6 +185,25 @@ class CriterionToOQL extends CriterionConversionAbstract
 							}
 						}
 					}
+					// search for "undefined"
+					for ($i = 0; $i < count($aValues); $i++)
+					{
+						$aValue = $aValues[$i];
+						if (isset($aValue['value']) && ($aValue['value'] === 'null'))
+						{
+							if ($oAttDef instanceof AttributeExternalKey)
+							{
+								$sFilterOnUndefined = "({$sRef} = 0)";
+							}
+							else
+							{
+								$sFilterOnUndefined = "ISNULL({$sRef})";
+							}
+							$bFilterOnUndefined = true;
+							unset($aValues[$i]);
+							break;
+						}
+					}
 				}
 			}
 		} catch (\CoreException $e)
@@ -195,6 +217,21 @@ class CriterionToOQL extends CriterionConversionAbstract
 		}
 		$sInList = implode("','", $aInValues);
 
+		if ($bFilterOnUndefined)
+		{
+			if (count($aValues) === 0)
+			{
+				return "{$sFilterOnUndefined}";
+			}
+
+			if (count($aInValues) == 1)
+			{
+				return "(({$sRef} = '$sInList') OR {$sFilterOnUndefined})";
+			}
+
+			return "({$sRef} IN ('$sInList') OR {$sFilterOnUndefined})";
+		}
+
 		if (count($aInValues) == 1)
 		{
 			return "({$sRef} = '$sInList')";