Cannot drop last unique index of an immediate materialized view

Error constant SQLE_DELETE_UPDATABLE_MATVIEW_INDEX_ERR
ODBC 2 State 42000
ODBC 3 State ERROR
Severity 16
SQLCODE –1184
SQLSTATE 55W21
Sybase error code 13331
Probable cause

You attempted to drop the last unique index on non-nullable columns of an immediate materialized view. Immediate materialized view needs at least one unique index on non-nullable columns in order to be valid.