diff --git a/skimage/util/unique.py b/skimage/util/unique.py
index a09fadd9..6034d9a4 100644
--- a/skimage/util/unique.py
+++ b/skimage/util/unique.py
@@ -30,6 +30,11 @@ def unique_rows(ar):
     if ar.ndim != 2:
         raise ValueError("unique_rows() only makes sense for 2D arrays, "
                          "got %dd" % ar.ndim)
+    # the view in the next line only works if the array is C-contiguous
+    ar = np.ascontiguousarray(ar)
+    # np.unique() finds identical items in a raveled array. To make it
+    # see each row as a single item, we create a view of each row as a
+    # byte string of length itemsize times number of columns in `ar`
     ar_row_view = ar.view('|S%d' % (ar.itemsize * ar.shape[1]))
     _, unique_row_indices = np.unique(ar_row_view, return_index=True)
     ar_out = ar[unique_row_indices]