diff --git a/skimage/morphology/convex_hull.py b/skimage/morphology/convex_hull.py
index ae395eac..bb9ca734 100644
--- a/skimage/morphology/convex_hull.py
+++ b/skimage/morphology/convex_hull.py
@@ -20,12 +20,12 @@ def convex_hull_image(image):
 
     Parameters
     ----------
-    image : ndarray
+    image : (M, N) array
         Binary input image. This array is cast to bool before processing.
 
     Returns
     -------
-    hull : ndarray of bool
+    hull : (M, N) array of bool
         Binary image with pixels in convex hull set to True.
 
     References
@@ -38,12 +38,9 @@ def convex_hull_image(image):
         raise ImportError("Could not import scipy.spatial.Delaunay, "
                           "only available in scipy >= 0.9.")
 
-    image = image.astype(bool)
-
     # Here we do an optimisation by choosing only pixels that are
     # the starting or ending pixel of a row or column.  This vastly
-    # limits the number of coordinates to examine for the virtual
-    # hull.
+    # limits the number of coordinates to examine for the virtual hull.
     coords = possible_hull(image.astype(np.uint8))
     N = len(coords)
 
diff --git a/skimage/morphology/tests/test_convex_hull.py b/skimage/morphology/tests/test_convex_hull.py
index 67850cf2..c0cc954e 100644
--- a/skimage/morphology/tests/test_convex_hull.py
+++ b/skimage/morphology/tests/test_convex_hull.py
@@ -139,5 +139,6 @@ def test_object():
 
     assert_raises(ValueError, convex_hull_object, image, 7)
 
+
 if __name__ == "__main__":
     np.testing.run_module_suite()