diff --git a/skimage/morphology/binary.py b/skimage/morphology/binary.py
index 5b019051..4612a35d 100644
--- a/skimage/morphology/binary.py
+++ b/skimage/morphology/binary.py
@@ -21,7 +21,7 @@ def binary_erosion(image, selem=None, out=None):
     selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
         If None, use cross-shaped structuring element (connectivity=1).
-    out : ndarray of bool
+    out : ndarray of bool, optional
         The array to store the result of the morphology. If None is
         passed, a new array will be allocated.
 
diff --git a/skimage/morphology/grey.py b/skimage/morphology/grey.py
index 2b4a01a8..565ec9a6 100644
--- a/skimage/morphology/grey.py
+++ b/skimage/morphology/grey.py
@@ -307,7 +307,7 @@ def black_tophat(image, selem=None, out=None):
     selem : ndarray, optional
         The neighborhood expressed as a 2-D array of 1's and 0's.
         If None, use cross-shaped structuring element (connectivity=1).
-    out : ndarray
+    out : ndarray, optional
         The array to store the result of the morphology. If None
         is passed, a new array will be allocated.
 
diff --git a/skimage/morphology/selem.py b/skimage/morphology/selem.py
index 4147d93b..36ce4fe9 100644
--- a/skimage/morphology/selem.py
+++ b/skimage/morphology/selem.py
@@ -310,4 +310,5 @@ def _default_selem(ndim):
         are 1 and 0 otherwise.
 
     """
-    return ndimage.morphology.generate_binary_structure(ndim, 1)
\ No newline at end of file
+    return ndimage.morphology.generate_binary_structure(ndim, 1)
+    
\ No newline at end of file