code: def test_subfield2():
      myfield = basic_field_random()
      valid = np.zeros(myfield.V)
      sf = myfield.subfield(valid)
      assert sf is None
  
imports: import numpy
  import numpy.random
  from numpy.testing import assert_array_equal
  from ..field import field_from_coo_matrix_and_data, field_from_graph_and_data
  from ..graph import wgraph_from_3d_grid
label_desc: Use image node_id 28 for calling a global function with example usage: test_subfield2() without return types