Every now and then I'm executing a macro in Excel/VBA, and I get an error message box with nothing but three digits, "400". No link to help, no source, no description, nothing but "400". What is this error?

Age 11, sixth grade, was when I was first exposed to BASIC. And I mean *really* *basic* BASIC. That was 1972, so we worked on TeleTypes with infinite rolls of Grade ZZZ paper and paper-tape punches to save our files. Later the school got a Linolex, which saved stuff on regular cassette tapes, then we got a Wang 2200, also with cassettes -- and a keyboard in alphabetical order!