diff --git a/PyCTBN/PyCTBN/structure_graph/network_generator.py b/PyCTBN/PyCTBN/structure_graph/network_generator.py
index 823bb13..57599ef 100644
--- a/PyCTBN/PyCTBN/structure_graph/network_generator.py
+++ b/PyCTBN/PyCTBN/structure_graph/network_generator.py
@@ -21,6 +21,9 @@ class NetworkGenerator(object):
         self._graph.add_edges(s.edges)
 
     def generate_cims(self, min_val, max_val):
+        if self._graph is None:
+            return
+
         self._cims = {}
 
         for i, l in enumerate(self._labels):
diff --git a/test_networkgenerator.py b/test_networkgenerator.py
new file mode 100644
index 0000000..d4f4882
--- /dev/null
+++ b/test_networkgenerator.py
@@ -0,0 +1,38 @@
+import unittest
+import random
+import numpy as np
+
+from PyCTBN.PyCTBN.structure_graph.network_generator import NetworkGenerator
+
+class TestNetworkGenerator(unittest.TestCase):
+    def test_generate_graph(self):
+        labels = ["U", "V", "W", "X", "Y", "Z"]
+        card = 3
+        vals = [card for l in labels]
+        ng = NetworkGenerator(labels, vals)
+        ng.generate_graph()
+        self.assertEqual(len(labels), len(ng.graph.nodes))
+        self.assertEqual(len([edge for edge in ng.graph.edges if edge[0] == edge[1]]), 0)
+
+    def test_generate_cims(self):
+        labels = ["U", "V", "W", "X", "Y", "Z"]
+        card = 3
+        vals = [card for l in labels]
+        cim_min = random.uniform(0.5, 5)
+        cim_max = random.uniform(0.5, 5) + cim_min
+        ng = NetworkGenerator(labels, vals)
+        ng.generate_graph()
+        ng.generate_cims(cim_min, cim_max)
+        self.assertEqual(len(ng.cims), len(labels))      
+        self.assertListEqual(list(ng.cims.keys()), labels)
+        for key in ng.cims:
+            p_card = ng.graph.get_ordered_by_indx_set_of_parents(key)[2]
+            comb = ng.graph.build_p_comb_structure_for_a_node(p_card)
+            self.assertEqual(len(ng.cims[key].actual_cims), len(comb))
+            for cim in ng.cims[key].actual_cims:
+                self.assertEqual(sum(c > 0 for c in cim.cim.diagonal()), 0)
+                for i, row in enumerate(cim.cim):
+                    self.assertEqual(round(sum(row) - row[i], 8), round(-1 * row[i], 8))
+                    self.assertEqual(sum(c < 0 for c in np.delete(cim.cim[i], i)), 0)
+    
+unittest.main()
\ No newline at end of file