diff -r e335eebdae5b -r a81f9cfc9775 src/lemon/graph_writer.h
--- a/src/lemon/graph_writer.h	Tue Apr 12 17:33:41 2005 +0000
+++ b/src/lemon/graph_writer.h	Tue Apr 12 17:34:51 2005 +0000
@@ -382,6 +382,7 @@
 
     void writeEdgeSet(WriterBase<Node>* nodeWriter, 
 		      WriterBase<Edge>* & edgeWriter) {
+      if (edge_map_writers.size() == 0) return;
       if (nodeWriter == 0) {
 	throw DataFormatError("Cannot find node id map");
       }
@@ -406,6 +407,7 @@
     }
 
     void writeNodes(WriterBase<Node>* nodeWriter) {
+      if (node_writers.size() == 0) return;
       if (nodeWriter == 0) {
 	throw DataFormatError("Cannot find node id map");
       }
@@ -418,6 +420,7 @@
     }
 
     void writeEdges(WriterBase<Edge>* edgeWriter) {
+      if (edge_writers.size() == 0) return;
       if (edgeWriter == 0) {
 	throw DataFormatError("Cannot find node id map");
       }