"Error creating foreign key (check data types)…" but they do match…

I'm getting the following error trying to create a constraint:


Error creating foreign key (check data types)

Problem is… those column data types do match.

The target is an int(10) unsigned NOT NULL and is the PRIMARY column.
The source, in the same table, is an int(10) unsigned DEFAULT NULL, and the constraint I'm trying to create is a simple ON DELETE RESTRICT, ON UPDATE CASCADE.

Now, of course the target is NOT NULL and the source is NULL, but that's entirely intentional and what I need, and I can't see how could this be a problem. I could understand if it was the other way around…

Is this really the problem? Why? How do I solve it, then? I don't want to be forced to insert a value there even when I don't need one, neither I want to enable NULL on the PRIMARY column, which doesn't make much sense.

What else could be the problem?

It was a bug with phpmyadmin.

When you create a new constraint you usually are allowed to leave its name field blank, and it fills it for you.

This time, for some still unknown reason, this didn't work and gave this unrelated error.

Filling the name field did the trick, and allowed me to create the constraint.

StackExchange Database Administrators Q#149516, answer score: 5