What does "Warning: No port … found in index" mean?

[Jim DeLaHunt]

Hi, folks:

In doing my daily `sudo port selfupdate` on 16 November UTC, I 
encountered a warning which is new to me:

"Warning: No port […portname…] found in index"

In redoing `sudo port selfupdate`, this warning did not reoccur.

What is the significance of this warning?  What, if anything, should I 
do about it?

The MacPorts output was something like this:

% sudo port selfupdate
Password:
Warning: No port libiconv found in the index.
Warning: No port libiconv found in the index.
Warning: No port expat found in the index.
Warning: No port libiconv found in the index.
--->  Fetching archive for jbig2dec
…[elided remaining 170 lines]…


The set of ports mentioned in the warnings were: expat, fribidi, 
libiconv, libunistring, libxml2, p11-kit, pango, python_select, 
python3_select, urw-fonts, xorg-libsm, xorg-libX11, xorg-libXaw, 
xorg-libxcb, xorg-libXcursor, xorg-libXext, xorg-libXfixes, xorg-libXi, 
xorg-libXinerama, xorg-libXrandr, xorg-libXScrnSaver, xorg-libXt, 
xorg-libXxf86vm, xorg-xcb-util, xpm, xrender.

I spot-checked a few of the ports. They are currently valid, installed 
ports, with one version active and one older version inactive.

What is this "index"?

Is this warning related to the upgrade of MacPorts base recently?

What is the significance of this warning?  What, if anything, should I 
do about it?

Thank you for your insight,
        —Jim DeLaHunt