diff --git a/src/treelua/toDot.lua b/src/treelua/toDot.lua
index 1a80e4e..03a1fe8 100644
--- a/src/treelua/toDot.lua
+++ b/src/treelua/toDot.lua
@@ -31,9 +31,13 @@ return function(graph)
     for n1, n2, edge in graph:iterPairs() do
         local n1_s = node2name[n1]
         local n2_s = node2name[n2]
-        local edge_s = edge.length or ''
-        local edge_str = '%s -- %s [label=%q];'
-        print(edge_str:format(n1_s, n2_s, edge_s))
+        if edge.length then
+            local edge_str = '%s -- %s [label=%q];'
+            print(edge_str:format(n1_s, n2_s, edge.length))
+        else
+            local edge_str = '%s -- %s;'
+            print(edge_str:format(n1_s, n2_s))
+        end
     end
     -- close graph
     print('}')