Index: /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java	(revision 11794)
+++ /trunk/src/org/openstreetmap/josm/io/session/SessionWriter.java	(revision 11795)
@@ -248,9 +248,13 @@
                 for (Layer depLayer : deps) {
                     int depIndex = layers.indexOf(depLayer);
-                    if (depIndex == -1)
-                        throw new AssertionError();
-                    depsInt.add(depIndex+1);
+                    if (depIndex == -1) {
+                        Main.warn("Unable to find " + depLayer);
+                    } else {
+                        depsInt.add(depIndex+1);
+                    }
                 }
-                el.setAttribute("depends", Utils.join(",", depsInt));
+                if (!depsInt.isEmpty()) {
+                    el.setAttribute("depends", Utils.join(",", depsInt));
+                }
             }
             layersEl.appendChild(el);