Index: applications/editors/josm/plugins/HouseNumberTaggingTool/src/org/openstreetmap/josm/plugins/housenumbertool/TagDialog.java
===================================================================
--- applications/editors/josm/plugins/HouseNumberTaggingTool/src/org/openstreetmap/josm/plugins/housenumbertool/TagDialog.java	(revision 34073)
+++ applications/editors/josm/plugins/HouseNumberTaggingTool/src/org/openstreetmap/josm/plugins/housenumbertool/TagDialog.java	(revision 34074)
@@ -359,14 +359,16 @@
 
     static String incrementHouseNumber(String number, int increment) {
-        try {
-            Matcher m = Pattern.compile("([^\\pN]+)?(\\pN+)([^\\pN]+)?").matcher(number);
-            if (m.matches()) {
-                String prefix = m.group(1) != null ? m.group(1) : "";
-                int n = Integer.valueOf(m.group(2)) + increment;
-                String suffix = m.group(3) != null ? m.group(3) : "";
-                return prefix + n + suffix;
-            }
-        } catch (NumberFormatException e)  {
-            // Do nothing
+        if (number != null) {
+            try {
+                Matcher m = Pattern.compile("([^\\pN]+)?(\\pN+)([^\\pN]+)?").matcher(number);
+                if (m.matches()) {
+                    String prefix = m.group(1) != null ? m.group(1) : "";
+                    int n = Integer.valueOf(m.group(2)) + increment;
+                    String suffix = m.group(3) != null ? m.group(3) : "";
+                    return prefix + n + suffix;
+                }
+            } catch (NumberFormatException e)  {
+                // Do nothing
+            }
         }
         return null;
Index: applications/editors/josm/plugins/HouseNumberTaggingTool/test/unit/org/openstreetmap/josm/plugins/housenumbertool/TagDialogTest.java
===================================================================
--- applications/editors/josm/plugins/HouseNumberTaggingTool/test/unit/org/openstreetmap/josm/plugins/housenumbertool/TagDialogTest.java	(revision 34073)
+++ applications/editors/josm/plugins/HouseNumberTaggingTool/test/unit/org/openstreetmap/josm/plugins/housenumbertool/TagDialogTest.java	(revision 34074)
@@ -3,4 +3,5 @@
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 
 import org.junit.Test;
@@ -22,4 +23,5 @@
         //assertEquals("۲", TagDialog.incrementHouseNumber("۱", 1)); // FIXME: how to increment persian numbers ?
         assertEquals("2", TagDialog.incrementHouseNumber("۱", 1));
+        assertNull(TagDialog.incrementHouseNumber(null, 1));
     }
 }