Fix for IndexError in metropolis discrete indexing

Author: Christopher Fonnesbeck

pymc3/step_methods/metropolis.py

@@ -91,7 +91,7 @@ def __init__(self, vars=None, S=None, proposal_dist=NormalProposal, scaling=1.,
         self.accepted = 0
 
         # Determine type of variables
-        self.discrete = np.hstack(np.ravel([[v.dtype in discrete_types ] * v.dsize for v in vars]))
+        self.discrete = np.ravel([[v.dtype in discrete_types ] * (v.dsize or 1) for v in vars])
         self.any_discrete = self.discrete.any()
         self.all_discrete = self.discrete.all()