Type error reported in mock file

This grammar contains a type error:

%token TOK
%start a
%type <unit> a
%%
let a :=
  | TOK
  | TOK*

The type error is unfortunately reported in the mock file:

File "file__mock.ml.mock", line 34, characters 5-17:
34 |     ((_1 : 'tv_a) : (
          ^^^^^^^^^^^^
Error: This expression has type unit list
       but an expression was expected of type unit

This problem was reported by yannl35133@gmail.com

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