Index: trunk/src/org/openstreetmap/josm/command/DeleteCommand.java
===================================================================
--- trunk/src/org/openstreetmap/josm/command/DeleteCommand.java	(revision 7674)
+++ trunk/src/org/openstreetmap/josm/command/DeleteCommand.java	(revision 7675)
@@ -44,5 +44,5 @@
 /**
  * A command to delete a number of primitives from the dataset.
- *
+ * @since 23
  */
 public class DeleteCommand extends Command {
@@ -60,9 +60,9 @@
      */
     public DeleteCommand(Collection<? extends OsmPrimitive> data) throws IllegalArgumentException {
-        if (data == null)
-            throw new IllegalArgumentException("Parameter 'data' must not be empty");
+        CheckParameterUtil.ensureParameterNotNull(data, "data");
         if (data.isEmpty())
             throw new IllegalArgumentException(tr("At least one object to delete required, got empty collection"));
         this.toDelete = data;
+        checkConsistency();
     }
 
@@ -74,6 +74,5 @@
      */
     public DeleteCommand(OsmPrimitive data) throws IllegalArgumentException {
-        CheckParameterUtil.ensureParameterNotNull(data, "data");
-        this.toDelete = Collections.singleton(data);
+        this(Collections.singleton(data));
     }
 
@@ -88,7 +87,5 @@
      */
     public DeleteCommand(OsmDataLayer layer, OsmPrimitive data) throws IllegalArgumentException {
-        super(layer);
-        CheckParameterUtil.ensureParameterNotNull(data, "data");
-        this.toDelete = Collections.singleton(data);
+        this(layer, Collections.singleton(data));
     }
 
@@ -104,9 +101,19 @@
     public DeleteCommand(OsmDataLayer layer, Collection<? extends OsmPrimitive> data) throws IllegalArgumentException{
         super(layer);
-        if (data == null)
-            throw new IllegalArgumentException("Parameter 'data' must not be empty");
+        CheckParameterUtil.ensureParameterNotNull(data, "data");
         if (data.isEmpty())
             throw new IllegalArgumentException(tr("At least one object to delete required, got empty collection"));
         this.toDelete = data;
+        checkConsistency();
+    }
+
+    private void checkConsistency() {
+        for (OsmPrimitive p : toDelete) {
+            if (p == null) {
+                throw new IllegalArgumentException("Primitive to delete must not be null");
+            } else if (p.getDataSet() == null) {
+                throw new IllegalArgumentException("Primitive to delete must be in a dataset");
+            }
+        }
     }
 
@@ -231,9 +238,6 @@
      * Delete the primitives and everything they reference.
      *
-     * If a node is deleted, the node and all ways and relations the node is part of are deleted as
-     * well.
-     *
+     * If a node is deleted, the node and all ways and relations the node is part of are deleted as well.
      * If a way is deleted, all relations the way is member of are also deleted.
-     *
      * If a way is deleted, only the way and no nodes are deleted.
      *
@@ -257,8 +261,33 @@
     }
 
+    /**
+     * Delete the primitives and everything they reference.
+     *
+     * If a node is deleted, the node and all ways and relations the node is part of are deleted as well.
+     * If a way is deleted, all relations the way is member of are also deleted.
+     * If a way is deleted, only the way and no nodes are deleted.
+     *
+     * @param layer the {@link OsmDataLayer} in whose context primitives are deleted. Must not be null.
+     * @param selection The list of all object to be deleted.
+     * @return command A command to perform the deletions, or null of there is nothing to delete.
+     * @throws IllegalArgumentException thrown if layer is null
+     */
     public static Command deleteWithReferences(OsmDataLayer layer, Collection<? extends OsmPrimitive> selection) {
         return deleteWithReferences(layer, selection, false);
     }
 
+    /**
+     * Try to delete all given primitives.
+     *
+     * If a node is used by a way, it's removed from that way. If a node or a way is used by a
+     * relation, inform the user and do not delete.
+     *
+     * If this would cause ways with less than 2 nodes to be created, delete these ways instead. If
+     * they are part of a relation, inform the user and do not delete.
+     *
+     * @param layer the {@link OsmDataLayer} in whose context the primitives are deleted
+     * @param selection the objects to delete.
+     * @return command a command to perform the deletions, or null if there is nothing to delete.
+     */
     public static Command delete(OsmDataLayer layer, Collection<? extends OsmPrimitive> selection) {
         return delete(layer, selection, true, false);
Index: trunk/src/org/openstreetmap/josm/data/osm/Way.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/Way.java	(revision 7674)
+++ trunk/src/org/openstreetmap/josm/data/osm/Way.java	(revision 7675)
@@ -285,4 +285,8 @@
             WayData wayData = (WayData) data;
 
+            if (!wayData.getNodes().isEmpty() && getDataSet() == null) {
+                throw new AssertionError("Data consistency problem - way without dataset detected");
+            }
+
             List<Node> newNodes = new ArrayList<>(wayData.getNodes().size());
             for (Long nodeId : wayData.getNodes()) {
@@ -290,6 +294,7 @@
                 if (node != null) {
                     newNodes.add(node);
-                } else
+                } else {
                     throw new AssertionError("Data consistency problem - way with missing node detected");
+                }
             }
             setNodes(newNodes);