--- a/python/ml.py	Wed Mar 08 17:46:28 2017 -0500
+++ b/python/ml.py	Fri Mar 10 15:31:26 2017 -0500
@@ -150,6 +150,10 @@
     TODO: at each step, optimize the prototype as the most similar in its current cluster (can be done easily if similarities are already computed)'''
 
     # sort instances based on length
+    if len(instances) == 0:
+        print('no instances to cluster (empty list)')
+        return None
+
     indices = range(len(instances))
     if randomInitialization:
         indices = np.random.permutation(indices)