Index: src/org/openstreetmap/josm/io/InvalidXmlCharacterFilter.java
===================================================================
--- src/org/openstreetmap/josm/io/InvalidXmlCharacterFilter.java	(revision 6078)
+++ src/org/openstreetmap/josm/io/InvalidXmlCharacterFilter.java	(working copy)
@@ -29,6 +29,7 @@
         for (int i = 0; i < INVALID_CHARS.length; ++i) {
             INVALID_CHARS[i] = true;
         }
+        INVALID_CHARS[0x0] = false; // NULL
         INVALID_CHARS[0x9] = false; // tab
         INVALID_CHARS[0xA] = false; // LF
         INVALID_CHARS[0xD] = false; // CR
@@ -58,7 +59,7 @@
     private byte filter(byte in) {
         if (in < 0x20 && in >= 0 && INVALID_CHARS[in]) {
             if (firstWarning) {
-                Main.warn("Invalid xml character encountered.");
+                Main.warn(String.format("Invalid xml character (0x%02x) encountered.", in));
                 firstWarning = false;
             }
             return 0x20;