Alter table add foreign key causes can't create table error

[Andrew Cutforth]

I am creating two tables then doing an alter table to add a foreign key constraint and it gives the following error:
Error Code: 1005. Can't create table 'mydb.#sql-870_16' (errno: 150)

Here is a simple test to prove it:

CREATE TABLE `test` (
`id` binary(16) NOT NULL,
`description` varchar(100) NOT NULL,
`main_contact` binary(16) NOT NULL,
PRIMARY KEY (`id`)
);

CREATE TABLE `test2` (
`id` binary(16) NOT NULL,
`description` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`)
);

-- This command fails
alter table test add constraint test_ri foreign key (main_contact) references test2;

Does anyone know what is wrong?

Thanks