ORA-15001: diskgroup "" does not exist or is not mounted

Issue:
====
SQL> drop diskgroup TESTDAT;
drop diskgroup TESTDAT
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15001: diskgroup "TESTDAT" does not exist or is not mounted


SQL> select name from v$asm_diskgroup;

NAME
------------------------------
TESTDAT


Fix:-
====
SQL> drop diskgroup TESTDAT force including contents;

Diskgroup dropped.