diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index 9eae22dbed..1eed118e0e 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -102,6 +102,7 @@ def image_to_data(img, compression_quality=95, fmt='JPEG', png_compression_level
     if is_jpeg:
         if img.hasAlphaChannel():
             img = blend_image(img)
+        # QImageWriter only gained the following options in Qt 5.5
         if jpeg_optimized and hasattr(QImageWriter, 'setOptimizedWrite'):
             w.setOptimizedWrite(True)
         if jpeg_progressive and hasattr(QImageWriter, 'setProgressiveScanWrite'):