In [127]: t=Table(); t.add_column(np.ma.masked_inside(np.arange(0,6),2,4).reshape((3,-1) ), name='arr')

In [128]: t
Out[128]: 
<Table length=3>
  arr   
int64[2]
--------
  0 .. 1
-- .. --
 -- .. 5

In [129]: t.add_index('arr')
/Users/jdsmith/code/python/astropy/astropy/table/index.py:92: MaskedArrayFutureWarning: In the future the default for argsort will be axis=-1, not the current None, to match its documentation and np.argsort. Explicitly pass -1 or None to silence this warning.
  row_index = Column(col.argsort())
Traceback (most recent call last):
  Input In [129] in <cell line: 1>
    t.add_index('arr')
  File ~/code/python/astropy/astropy/table/table.py:1016 in add_index
    index = Index(columns, engine=engine, unique=unique)
  File ~/code/python/astropy/astropy/table/index.py:93 in __init__
    data = Table([col[row_index]])
  File astropy/table/_column_mixins.pyx:87 in astropy.table._column_mixins._MaskedColumnGetitemShim.__getitem__
  File astropy/table/_column_mixins.pyx:57 in astropy.table._column_mixins.base_getitem
  File astropy/table/_column_mixins.pyx:81 in astropy.table._column_mixins.masked_column_getitem
  File /usr/local/lib/python3.10/site-packages/numpy/ma/core.py:3222 in __getitem__
    dout = self.data[indx]
  File /usr/local/lib/python3.10/site-packages/numpy/ma/core.py:3222 in __getitem__
    dout = self.data[indx]
IndexError: index 5 is out of bounds for axis 0 with size 3