diff --git a/src/PIL/ImageChops.py b/src/PIL/ImageChops.py
index 89016730e..422811351 100644
--- a/src/PIL/ImageChops.py
+++ b/src/PIL/ImageChops.py
@@ -54,7 +54,7 @@ def invert(image):
 def lighter(image1, image2):
     """
     Compares the two images, pixel by pixel, and returns a new image containing
-    the lighter values.
+    the lighter values. Note that at least one of the images must have mode "1".
 
     .. code-block:: python
 
@@ -70,8 +70,8 @@ def lighter(image1, image2):
 
 def darker(image1, image2):
     """
-    Compares the two images, pixel by pixel, and returns a new image
-    containing the darker values.
+    Compares the two images, pixel by pixel, and returns a new image containing
+    the darker values. Note that at least one of the images must have mode "1".
 
     .. code-block:: python
 
@@ -88,7 +88,7 @@ def darker(image1, image2):
 def difference(image1, image2):
     """
     Returns the absolute value of the pixel-by-pixel difference between the two
-    images.
+    images. At least one of the images must be "1" mode.
 
     .. code-block:: python
 
@@ -107,7 +107,8 @@ def multiply(image1, image2):
     Superimposes two images on top of each other.
 
     If you multiply an image with a solid black image, the result is black. If
-    you multiply with a solid white image, the image is unaffected.
+    you multiply with a solid white image, the image is unaffected. At least
+    one of the images must be "1" mode.
 
     .. code-block:: python
 
@@ -123,7 +124,8 @@ def multiply(image1, image2):
 
 def screen(image1, image2):
     """
-    Superimposes two inverted images on top of each other.
+    Superimposes two inverted images on top of each other. At least one of the
+    images must be "1" mode.
 
     .. code-block:: python
 
@@ -141,6 +143,7 @@ def add(image1, image2, scale=1.0, offset=0):
     """
     Adds two images, dividing the result by scale and adding the
     offset. If omitted, scale defaults to 1.0, and offset to 0.0.
+    At least one of the images must be "1" mode.
 
     .. code-block:: python
 
@@ -156,8 +159,9 @@ def add(image1, image2, scale=1.0, offset=0):
 
 def subtract(image1, image2, scale=1.0, offset=0):
     """
-    Subtracts two images, dividing the result by scale and adding the
-    offset. If omitted, scale defaults to 1.0, and offset to 0.0.
+    Subtracts two images, dividing the result by scale and adding the offset.
+    If omitted, scale defaults to 1.0, and offset to 0.0. At least one of the
+    images must be "1" mode.
 
     .. code-block:: python
 
@@ -172,7 +176,8 @@ def subtract(image1, image2, scale=1.0, offset=0):
 
 
 def add_modulo(image1, image2):
-    """Add two images, without clipping the result.
+    """Add two images, without clipping the result. At least one of the images
+    must be "1" mode.
 
     .. code-block:: python
 
@@ -187,7 +192,8 @@ def add_modulo(image1, image2):
 
 
 def subtract_modulo(image1, image2):
-    """Subtract two images, without clipping the result.
+    """Subtract two images, without clipping the result. At least one of the
+    images must be "1" mode.
 
     .. code-block:: python
 
@@ -202,7 +208,8 @@ def subtract_modulo(image1, image2):
 
 
 def logical_and(image1, image2):
-    """Logical AND between two images.
+    """Logical AND between two images. At least one of the images must be "1"
+    mode.
 
     .. code-block:: python
 
@@ -217,7 +224,8 @@ def logical_and(image1, image2):
 
 
 def logical_or(image1, image2):
-    """Logical OR between two images.
+    """Logical OR between two images. At least one of the images must be "1"
+    mode.
 
     .. code-block:: python
 
@@ -232,7 +240,8 @@ def logical_or(image1, image2):
 
 
 def logical_xor(image1, image2):
-    """Logical XOR between two images.
+    """Logical XOR between two images. At least one of the images must be "1"
+    mode.
 
     .. code-block:: python