diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index b5323d252..b359f7259 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -474,8 +474,8 @@ def _write_multiple_frames(im, fp, palette):
             im_frames.append({"im": im_frame, "bbox": bbox, "encoderinfo": encoderinfo})
     
     # see: https://github.com/python-pillow/Pillow/issues/4002
-    if len(im_frames) == 1 and 'duration' in im_frames[0]['encoderinfo']:
-        im.encoderinfo['duration'] = im_frames[0]['encoderinfo']['duration']
+    if len(im_frames) == 1 and "duration" in im_frames[0]["encoderinfo"]:
+        im.encoderinfo["duration"] = im_frames[0]["encoderinfo"]["duration"]
 
     if len(im_frames) > 1:
         for frame_data in im_frames: