Cannot alter a column in an index

Error constant SQLE_COLUMN_IN_INDEX
ODBC 2 State ERROR
ODBC 3 State ERROR
Severity 16
SQLCODE –127
SQLSTATE 53W05
Sybase error code 4939
Probable cause

You may have attempted to delete or modify the definition of a column that is part of a primary or foreign key. You may have attempted to delete a column that has an index on it. In this case, DROP the index or key, perform the ALTER statement, and then add the index or key again.