diff --git a/include/nuttx/fb.h b/include/nuttx/fb.h
index 4b806825b06..8a514688725 100644
--- a/include/nuttx/fb.h
+++ b/include/nuttx/fb.h
@@ -209,6 +209,9 @@ struct fb_planeinfo_s
 struct fb_cmap_s
 {
  uint16  len;              /* Number of color entries */
+
+ /* Each color component.  Any may be NULL if not used */
+
  uint16 *red;              /* Table of red values */
  uint16 *green;            /* Table of red values */
  uint16 *blue;             /* Table of red values */