code: def test_subpix_no_class():
      img = np.zeros((50, 50))
      subpix = corner_subpix(img, np.array([[25, 25]]))
      assert_array_equal(subpix[0], (np.nan, np.nan))
  
      img[25, 25] = 1e-10
      corner = peak_local_max(
          corner_harris(img),
          min_distance=10,
          threshold_rel=0,
          num_peaks=1,
      )
      subpix = corner_subpix(img, corner)
      assert_array_equal(subpix[0], (np.nan, np.nan))
  
imports: import numpy
  import pytest
  from numpy.testing import assert_almost_equal, assert_array_equal, assert_equal
  from skimage import data, draw, img_as_float
  from skimage._shared._warnings import expected_warnings
  from skimage._shared.testing import run_in_parallel
  from skimage._shared.utils import _supported_float_type
  from skimage.color import rgb2gray
  from skimage.feature import corner_fast, corner_foerstner, corner_harris, corner_kitchen_rosenfeld, corner_moravec, corner_orientations, corner_peaks, corner_shi_tomasi, corner_subpix, hessian_matrix, hessian_matrix_det, hessian_matrix_eigvals, peak_local_max, shape_index, structure_tensor, structure_tensor_eigenvalues
  from skimage.morphology import cube, octagon
label_desc: Use image node_id 26 for calling a global function with example usage: test_subpix_no_class() without return types