Foreign key error 1091

[Fred Riley]

I've just now tried to add a field to the structure of a table (called 'RLO') and I get the error:

ERROR 1091: Can't DROP 'RLO_ibfk_1'. Check that column/key exists

I've searched the MySQL manual for this error but there's no further detail as far as I can see, other than to mention that it's a foreign key error. I also get the error if I try to make any other structural changes, for instance if I try to change the data type of a field. The primary key of the table, rlo_num, is a foreign key in a couple of other junction tables, so presumably that's causing the problem, but I'm not trying to DROP anything, honest guv. Is it the case that once you've employed a primary key as a foreign key elsewhere that the primary key's table is set in stone and can't be changed in any way?

Could someone please point me in the right direction on this problem? TIA.

Cheers

Fred
http://www.nottingham.ac.uk/nursing/staff/support-staff/fred_riley.html