Freeze?

When I try to save a modifed FORTRAN77 source file named prog.F, I get a
message in the minibuffer that says "write prog.F using freeze (y or n)?".
There are no visible non-ascii characters in the file buffer--just plain old
FORTRAN.

When I say, y, it replies "Encoding... Encoding failed" and the file is not
saved. When I say n, the file is saved as I'm used to.

If I rename the file to prog.f, the odd behavior doesn't happen.

What is happening and what does the freeze refer to?  I am using the
December sumo package release and xemacs version 21.4.19.

Thanks.

Tom Browder
Niceville, Florida