dune subst: internal Sys_error raised

[avsm]

$ dune subst
Error: exception Sys_error("Is a directory")
Backtrace:
Raised by primitive operation at file "stdlib.ml", line 412, characters 12-37
Called from file "stdlib.ml", line 425, characters 2-25
Called from file "src/stdune/exn.ml", line 27, characters 8-11
Re-raised at file "src/stdune/exn.ml", line 29, characters 30-37
Called from file "src/stdune/io.ml" (inlined), line 94, characters 29-64
Called from file "src/watermarks.ml", line 153, characters 10-27
Called from file "list.ml", line 106, characters 12-15
Called from file "src/watermarks.ml", line 261, characters 2-117
Called from file "src/fiber/fiber.ml", line 95, characters 6-13

I must not segfault.  Uncertainty is the mind-killer.  Exceptions are
the little-death that brings total obliteration.  I will fully express
my cases.  Execution will pass over me and through me.  And when it
has gone past, I will unwind the stack along its path.  Where the
cases are handled there will be nothing.  Only I will remain.
yak:opam2web avsm$ dune --version
1.8.2

[avsm]

This looks like something to do with submodules. There is an empty content/doc folder in opam2web and deleting that with this diff fixed the error:

commit 4f997d91cb878e35807ef45eece00422205d53a3 (HEAD -> work-with-ocamlorg)
Author: Anil Madhavapeddy <anil@recoil.org>
Date:   Sun Mar 31 18:37:33 2019 +0100

    remove empty directory

diff --git a/content/doc b/content/doc
deleted file mode 160000
index 13a77b2..0000000
--- a/content/doc
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 13a77b2bc576763bfa469ca3ef6f139402b7357b