diff --git a/remote-api/src/main/java/org/alfresco/rest/api/search/model/FacetField.java b/remote-api/src/main/java/org/alfresco/rest/api/search/model/FacetField.java
index 409bc811e1..8d5ac4d0c1 100644
--- a/remote-api/src/main/java/org/alfresco/rest/api/search/model/FacetField.java
+++ b/remote-api/src/main/java/org/alfresco/rest/api/search/model/FacetField.java
@@ -93,7 +93,7 @@ public class FacetField
 
     public String toFilterQuery(String value)
     {
-        return field+":\""+value+"\"";
+        return ("Null".equals(value)) ? "ISNULL:\"" + field + "\"" : field + ":\"" + value + "\"";
     }
 
     public String getPrefix()