diff --git a/deepwalk/graph.py b/deepwalk/graph.py
index c780799cc7ff5e7ef87e43d109fdf377f5ce0b65..428deca5586b0ee0476dd228efa5ecce44aa59e3 100644
--- a/deepwalk/graph.py
+++ b/deepwalk/graph.py
@@ -50,7 +50,7 @@ class Graph(defaultdict):
   
     t0 = time()
 
-    for v in self.keys():
+    for v in list(self):
       for other in self[v]:
         if v != other:
           self[other].append(v)