C-x C-f any_name.opam causes a "File mode specification error"

[gav451]

When I visit any file with an opam extension, I get the message:
File mode specification error: (void-variable tuareg-opam--flymake-proc-allowed-file-name-masks).
The file opens, but I do not know in what state the Tuareg-opam buffer is.

[hawnzug]

(require 'flymake-proc) or apply #310.