Postgresql 9.3 Postgis 2.1

Hello,

I have a problem.

When I execute this command;

alter TABLE ada alter column poly type geometry(GEOMETRY);

or

alter TABLE ada alter column poly type geometry(geometry,32768);

I get this message:

ERROR:  Geometry has Z dimension but column does not

********** Error **********

ERROR: Geometry has Z dimension but column does not

SQL state: 22023

how I solve this problem;

Could you help me?

Thanks

Your sincerely.