rndc reconfig: Unexpected end of input

[J David]

After recently improving the tracking of errors coming from commands
running from scripts, we found that a large number of “rndc reconfig”
requests (about 15-20% of all requests) error out with exit status 1
and the message:

rndc: ‘reconfig' failed: unexpected end of input

The “unexpected end of input” error is one that rndc usually issues if
a parameter is missing.  For example, “rndc refresh” without providing
a zone name on the command line.  But “rndc reconfig” doesn’t have any
additional command line parameters.

In this case, the rndc reconfig is issued after adding a zone file to
the configuration.

This is on BIND 9.12.2-P1 on FreeBSD 11.2, if that’s relevant.

Does anyone know what might be causing this error message?

Thanks for any advice!