typmod for custom type

I have created a custom type called my_numeric, which is roughly the same as the existing numeric type. I have declared in, out, typmod_in, and typmod_out functions. I create a table like:

CREATE TABLE test (col1 my_numeric(6, 3));

And then do \d test and can see that col1 is declared as (6, 3), which I think confirms that my typmod_in and typmod_out function are correct.

But when I do:

INSERT INTO test VALUES ('12.34');

It behaves as though the typmod parameter to my in function (PG_GETARG_INT32(2)) is passed in as -1. The docs (http://www.postgresql.org/docs/9.3/static/sql-createtype.html) says of the arguments that "the third is the typmod of the destination column, if known (-1 will be passed if not).".

Am I doing something wrong? Don't I need to know the typmod of the destination column so I can confirm that the format of the string being passed in conforms to the type modifier specified (eg so I can reject '1234.5678' for col1 with an overflow error)?

Thanks

James