Index: /applications/editors/josm/plugins/terracer/src/terracer/TerracerAction.java
===================================================================
--- /applications/editors/josm/plugins/terracer/src/terracer/TerracerAction.java	(revision 21162)
+++ /applications/editors/josm/plugins/terracer/src/terracer/TerracerAction.java	(revision 21163)
@@ -239,7 +239,9 @@
 		        // add the tags of the outline to each building (e.g. source=*)
 		        TagCollection.from(outline).applyTo(terr);
-				
-				String number = Integer.toString(from + i * step);
-
+
+				String number = null;
+				if (from != null) {
+					number = Integer.toString(from + i * step);
+				}
 		       	terr = addressBuilding(terr, street, streetName, number);