diff --git a/openbis_api/source/java/ch/systemsx/cisd/openbis/generic/shared/api/v1/dto/Sample.java b/openbis_api/source/java/ch/systemsx/cisd/openbis/generic/shared/api/v1/dto/Sample.java
index cdfc01155af67c5c20ee08803b1bb87fd60e9318..c4f6ce86e5e85b477953661882803dd3dee29c3b 100644
--- a/openbis_api/source/java/ch/systemsx/cisd/openbis/generic/shared/api/v1/dto/Sample.java
+++ b/openbis_api/source/java/ch/systemsx/cisd/openbis/generic/shared/api/v1/dto/Sample.java
@@ -438,15 +438,25 @@ public final class Sample implements Serializable, IIdentifierHolder
         if (retrievedFetchOptions.contains(SampleFetchOption.PROPERTIES))
         {
             builder.append(getProperties());
+        } else
+        {
+            builder.append("properties=?");
         }
         if (retrievedFetchOptions.contains(SampleFetchOption.PARENTS))
         {
             builder.append("parents", getParents());
+        } else
+        {
+            builder.append("parents=?");
         }
         if (retrievedFetchOptions.contains(SampleFetchOption.CHILDREN))
         {
             builder.append("children", getChildren());
         }
+        else
+        {
+            builder.append("children=?");
+        }
         return builder.toString();
     }