diff --git a/skimage/draw/tests/test_draw3d.py b/skimage/draw/tests/test_draw3d.py
index 2e1198eb..48caf1a0 100644
--- a/skimage/draw/tests/test_draw3d.py
+++ b/skimage/draw/tests/test_draw3d.py
@@ -1,9 +1,25 @@
 import numpy as np
 from numpy.testing import assert_array_equal, assert_allclose
+from nose.tools import raises
 
 from skimage.draw import ellipsoid, ellipsoid_stats
 
 
+@raises(ValueError)
+def test_ellipsoid_sign_parameters1():
+    ellipsoid(-1, 2, 2)
+
+
+@raises(ValueError)
+def test_ellipsoid_sign_parameters2():
+    ellipsoid(0, 2, 2)
+
+
+@raises(ValueError)
+def test_ellipsoid_sign_parameters3():
+    ellipsoid(-3, -2, 2)
+
+
 def test_ellipsoid_bool():
     test = ellipsoid(2, 2, 2)[1:-1, 1:-1, 1:-1]
     test_anisotropic = ellipsoid(2, 2, 4, spacing=(1., 1., 2.))
@@ -86,18 +102,18 @@ def test_ellipsoid_levelset():
 def test_ellipsoid_stats():
     # Test comparison values generated by Wolfram Alpha
     vol, surf = ellipsoid_stats(6, 10, 16)
-    assert(round(1280 * np.pi, 4) == round(vol, 4))
-    assert(1383.28 == round(surf, 2))
+    assert_allclose(1280 * np.pi, vol, atol=1e-4)
+    assert_allclose(1383.28, surf, atol=1e-2)
 
     # Test when a <= b <= c does not hold
     vol, surf = ellipsoid_stats(16, 6, 10)
-    assert(round(1280 * np.pi, 4) == round(vol, 4))
-    assert(1383.28 == round(surf, 2))
+    assert_allclose(1280 * np.pi, vol, atol=1e-4)
+    assert_allclose(1383.28, surf, atol=1e-2)
 
     # Larger test to ensure reliability over broad range
     vol, surf = ellipsoid_stats(17, 27, 169)
-    assert(round(103428 * np.pi, 4) == round(vol, 4))
-    assert(37426.3 == round(surf, 1))
+    assert_allclose(103428 * np.pi, vol, atol=1e-4)
+    assert_allclose(37426.3, surf, atol=1e-1)
 
 
 if __name__ == "__main__":
diff --git a/skimage/util/shape.py b/skimage/util/shape.py
index f91286c3..95e0a42e 100644
--- a/skimage/util/shape.py
+++ b/skimage/util/shape.py
@@ -111,9 +111,10 @@ def view_as_windows(arr_in, window_shape, step=1):
     window_shape: tuple
         Defines the shape of the elementary n-dimensional orthotope
         (better know as hyperrectangle [1]_) of the rolling window view.
-    step : int
+    step : int, optional
         Number of elements to skip when moving the window forward (by
-        default, move forward by one).
+        default, move forward by one). The value must be equal or larger
+        than one.
 
     Returns
     -------
diff --git a/skimage/util/tests/test_shape.py b/skimage/util/tests/test_shape.py
index b6975d0f..b66ac2e2 100644
--- a/skimage/util/tests/test_shape.py
+++ b/skimage/util/tests/test_shape.py
@@ -6,41 +6,35 @@ from skimage.util.shape import view_as_blocks, view_as_windows
 
 @raises(TypeError)
 def test_view_as_blocks_block_not_a_tuple():
-
     A = np.arange(10)
     view_as_blocks(A, [5])
 
 
 @raises(ValueError)
 def test_view_as_blocks_negative_shape():
-
     A = np.arange(10)
     view_as_blocks(A, (-2,))
 
 
 @raises(ValueError)
 def test_view_as_blocks_block_too_large():
-
     A = np.arange(10)
     view_as_blocks(A, (11,))
 
 
 @raises(ValueError)
 def test_view_as_blocks_wrong_block_dimension():
-
     A = np.arange(10)
     view_as_blocks(A, (2, 2))
 
 
 @raises(ValueError)
 def test_view_as_blocks_1D_array_wrong_block_shape():
-
     A = np.arange(10)
     view_as_blocks(A, (3,))
 
 
 def test_view_as_blocks_1D_array():
-
     A = np.arange(10)
     B = view_as_blocks(A, (5,))
     assert_equal(B, np.array([[0, 1, 2, 3, 4],
@@ -48,7 +42,6 @@ def test_view_as_blocks_1D_array():
 
 
 def test_view_as_blocks_2D_array():
-
     A = np.arange(4 * 4).reshape(4, 4)
     B = view_as_blocks(A, (2, 2))
     assert_equal(B[0, 1], np.array([[2, 3],
@@ -57,7 +50,6 @@ def test_view_as_blocks_2D_array():
 
 
 def test_view_as_blocks_3D_array():
-
     A = np.arange(4 * 4 * 6).reshape(4, 4, 6)
     B = view_as_blocks(A, (1, 2, 2))
     assert_equal(B.shape, (4, 2, 3, 1, 2, 2))
@@ -69,41 +61,40 @@ def test_view_as_blocks_3D_array():
 
 @raises(TypeError)
 def test_view_as_windows_input_not_array():
-
     A = [1, 2, 3, 4, 5]
     view_as_windows(A, (2,))
 
 
 @raises(TypeError)
 def test_view_as_windows_window_not_tuple():
-
     A = np.arange(10)
     view_as_windows(A, [2])
 
 
 @raises(ValueError)
 def test_view_as_windows_wrong_window_dimension():
-
     A = np.arange(10)
     view_as_windows(A, (2, 2))
 
 
 @raises(ValueError)
 def test_view_as_windows_negative_window_length():
-
     A = np.arange(10)
     view_as_windows(A, (-1,))
 
 
 @raises(ValueError)
 def test_view_as_windows_window_too_large():
-
     A = np.arange(10)
     view_as_windows(A, (11,))
 
 
-def test_view_as_windows_1D():
+@raises(ValueError)
+def test_view_as_windows_step_below_one():
+    A = np.arange(10)
+    view_as_windows(A, (11,), step=0.9)
 
+def test_view_as_windows_1D():
     A = np.arange(10)
     window_shape = (3,)
     B = view_as_windows(A, window_shape)
@@ -118,7 +109,6 @@ def test_view_as_windows_1D():
 
 
 def test_view_as_windows_2D():
-
     A = np.arange(5 * 4).reshape(5, 4)
     window_shape = (4, 3)
     B = view_as_windows(A, window_shape)