added type-checking enforcing that gradients are not integers

theano/gradient.py

@@ -776,6 +776,17 @@ def _populate_grad_dict(var_to_node_to_idx,
                              ' a gradient term, '
                             'this is prohibited') % node.op)
 
+                if not isinstance(term.type,
+                        (NullType,DisconnectedType)):
+                    assert hasattr(term,'dtype')
+                    if term.dtype.find('float') == -1:
+                        raise TypeError(str(node.op)+'.grad illegally '
+                                ' returned an integer-valued variable.'
+                                ' (Input index %d, dtype %s' % (i,
+                                    term.dtype))
+
+
+
             #cache the result
             term_dict[node] = input_grads