diff --git a/graphkit/network.py b/graphkit/network.py
index 0df3ddf8..60f08858 100644
--- a/graphkit/network.py
+++ b/graphkit/network.py
@@ -73,6 +73,9 @@ def add_op(self, operation):
         # assert layer is only added once to graph
         assert operation not in self.graph.nodes(), "Operation may only be added once"
 
+        ## Invalidate old plans.
+        self._cached_execution_plans = {}
+
         # add nodes and edges to graph describing the data needs for this layer
         for n in operation.needs:
             self.graph.add_edge(DataPlaceholderNode(n), operation)
diff --git a/setup.py b/setup.py
index bd7883f4..d3dfec84 100644
--- a/setup.py
+++ b/setup.py
@@ -28,7 +28,10 @@
      author_email='huyng@yahoo-inc.com',
      url='http://github.com/yahoo/graphkit',
      packages=['graphkit'],
-     install_requires=['networkx'],
+     install_requires=[
+          "networkx; python_version >= '3.5'",
+          "networkx == 2.2; python_version < '3.5'",
+     ],
      extras_require={
           'plot': ['pydot', 'matplotlib']
      },