Better error message.

theano/tensor/opt.py

@@ -1027,8 +1027,8 @@ class ShapeFeature(object):
                 # but this works with `local_useless_subtensor`, so for now we
                 # keep it this way. See #266 for a better long-term fix.
                 if getattr(d, 'dtype', 'int64') != 'int64':
-                    assert d.dtype in theano.tensor.discrete_dtypes, d.dtype
-                    assert str(d.dtype) != 'uint64'
+                    assert d.dtype in theano.tensor.discrete_dtypes, (node, d.dtype)
+                    assert str(d.dtype) != 'uint64', node
                     new_shape += sh[len(new_shape):i + 1]
                     new_shape[i] = theano.tensor.cast(d, 'int64')
             if new_shape: