diff --git a/PIL/Image.py b/PIL/Image.py
index 31d25421c..177f804f7 100644
--- a/PIL/Image.py
+++ b/PIL/Image.py
@@ -2302,8 +2302,8 @@ def alpha_composite(im1, im2):
     """
     Alpha composite im2 over im1.
 
-    :param im1: The first image.
-    :param im2: The second image.  Must have the same mode and size as
+    :param im1: The first image. Must have mode RGBA.
+    :param im2: The second image.  Must have mode RGBA, and the same size as
        the first image.
     :returns: An :py:class:`~PIL.Image.Image` object.
     """