Bug: Fix label sequence in regionprops.

Closes gh-806.

skimage/measure/_regionprops.py

@@ -503,6 +503,9 @@ def regionprops(label_image, properties=None,
 
     objects = ndimage.find_objects(label_image)
     for i, sl in enumerate(objects):
+        if sl is None:
+            continue
+        
         label = i + 1
 
         props = _RegionProperties(sl, label, label_image,

skimage/measure/tests/test_regionprops.py

@@ -347,6 +347,20 @@ def test_old_dict_interface():
     assert_equal(len(feats[0]), 8)
 
 
+def test_label_sequence():
+    a = np.empty((2, 2), dtype=np.int)
+    a[:, :] = 2
+    ps = regionprops(a)
+    assert len(ps) == 1
+    assert ps[0].label == 2
+
+
+def test_pure_background():
+    a = np.zeros((2, 2), dtype=np.int)
+    ps = regionprops(a)
+    assert len(ps) == 0
+
+
 if __name__ == "__main__":
     from numpy.testing import run_module_suite
     run_module_suite()