Not_found in driver on vhd-tool

[avsm]

Generating documentation for vhd-tool
Fatal error: exception Not_found
Raised at file "list.ml", line 179, characters 16-25
Called from file "src/opam-doc-index/index.ml", line 84, characters 3-70
Called from file "list.ml", line 84, characters 24-34
Called from file "src/opam-doc-index/index.ml", line 107, characters 15-57
Called from file "src/opam-doc-index/driver.ml", line 159, characters 15-45