diff --git a/de.hftstuttgart.cityunits.model/model/Quantities.genmodel b/de.hftstuttgart.cityunits.model/model/Quantities.genmodel
index 1b3b361822d09a4bd23433e31aa30855721d323e..2571119b4758bc9a535676b4a77a7ad83e505176 100644
--- a/de.hftstuttgart.cityunits.model/model/Quantities.genmodel
+++ b/de.hftstuttgart.cityunits.model/model/Quantities.genmodel
@@ -7,7 +7,9 @@
   <foreignModel>Quantities.ecore</foreignModel>
   <genPackages prefix="Quantities" basePackage="de.hftstuttgart.cityunits.model" disposableProviderFactory="true"
       ecorePackage="Quantities.ecore#/">
-    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityDouble"/>
-    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityLong"/>
+    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityDouble" create="return de.hftstuttgart.units.NullableQuantity.create(it);"
+        convert="return it == null ? null : it.toString();"/>
+    <genDataTypes ecoreDataType="Quantities.ecore#//QuantityLong" create="return de.hftstuttgart.units.NullableQuantity.create(it);"
+        convert="return it == null ? null : it.toString();"/>
   </genPackages>
 </genmodel:GenModel>