why solveset and dsolve do not have _fields for ast.walk?

[meInvent bbird]

Traceback (most recent call last):
  File "astgraphforsympy.py", line 86, in <module>
    print get_func_calls(tree)
  File "astgraphforsympy.py", line 48, in get_func_calls
    for node in ast.walk(tree):
  File "C:\Python27\lib\ast.py", line 213, in walk
    todo.extend(iter_child_nodes(node))
  File "C:\Python27\lib\ast.py", line 178, in iter_child_nodes
    for name, field in iter_fields(node):
  File "C:\Python27\lib\ast.py", line 166, in iter_fields
    for field in node._fields:
AttributeError: Module instance has no attribute '_fields'