Index: /trunk/src/org/openstreetmap/josm/data/projection/CustomProjection.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/projection/CustomProjection.java	(revision 9105)
+++ /trunk/src/org/openstreetmap/josm/data/projection/CustomProjection.java	(revision 9106)
@@ -391,7 +391,5 @@
             return parseToWGS84(towgs84, ellps);
 
-        if (parameters.containsKey(Param.no_defs.key))
-            throw new ProjectionConfigurationException(tr("Datum required (+datum=*, +towgs84=* or +nadgrids=*)"));
-        return new CentricDatum(null, null, ellps);
+        return new NullDatum(null, ellps);
     }