diff --git a/docs/handbook/image-file-formats.rst b/docs/handbook/image-file-formats.rst
index fe310df6443..38c00f8702c 100644
--- a/docs/handbook/image-file-formats.rst
+++ b/docs/handbook/image-file-formats.rst
@@ -266,9 +266,12 @@ following options are available::
     :py:class:`PIL.ImagePalette.ImagePalette` object.
 
 **optimize**
-    If present and true, attempt to compress the palette by
-    eliminating unused colors. This is only useful if the palette can
-    be compressed to the next smaller power of 2 elements.
+    Whether to attempt to compress the palette by eliminating unused colors.
+    This is attempted by default, unless a palette is specified as an option or
+    as part of the first image's :py:attr:`~PIL.Image.Image.info` dictionary.
+
+    This is only useful if the palette can be compressed to the next smaller
+    power of 2 elements.
 
 Note that if the image you are saving comes from an existing GIF, it may have
 the following properties in its :py:attr:`~PIL.Image.Image.info` dictionary.
diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index bdea02005e7..0793d4b4277 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -656,7 +656,7 @@ def _save(im, fp, filename, save_all=False):
         palette = im.encoderinfo.get("palette", im.info.get("palette"))
     else:
         palette = None
-        im.encoderinfo["optimize"] = im.encoderinfo.get("optimize", True)
+        im.encoderinfo.setdefault("optimize", True)
 
     if not save_all or not _write_multiple_frames(im, fp, palette):
         _write_single_frame(im, fp, palette)