Cannot drop primary key or add primary key This db2 query:

alter table mylib.mytable add primary key(field1, field2, field3)


returns 


  [SQL0624] Table MYTABLE in MYLIB already has a primary key'.


However

alter table mylib.mytable drop primary key


returns:


  [SQL0539] Table MYTABLEin MYLIB does not have a primary or unique key'.


How can I remove the existing primary key and create the new one?

The file was created in green screen, not by executing a SQL CREATE TABLE. The database version is V7R2.
You can find all FKEY/PKEY/CHECK Constraints in QSYS2.TABLE_CONSTRAINTS to see and/or remove any keys built on the table.