diff --git a/Source/NSOutlineView.m b/Source/NSOutlineView.m
index f434f1be0..6bcb15c34 100644
--- a/Source/NSOutlineView.m
+++ b/Source/NSOutlineView.m
@@ -1079,6 +1079,7 @@ static NSImage *unexpandable  = nil;
        * item presumably), or perhaps we should treat this as
        * cancelling the drop?
        */
+	NSLog(@"Alert: Invalid drop item %@", item);
       return;
     }
   currentDropItem = item;