[tex4ht] what would cause " Can't close file `index.tmp' (file is not open)" --- error --- Illegal storage address

[Karl Berry]

    --- warning --- Can't close file `index.tmp' (file is not open)
    --- error --- Illegal storage address
    Make4ht: Fatal error. Command tex4ht returned exit code 256

That "Illegal storage address" is surely a weird error no matter what. I
will try to take a look and see if I can discern a fix. Glad there is a
workaround in the meantime. -k