// requireNonNull is safe because of the containsColumn check.
      return containsColumn(key) ? column((C) requireNonNull(key)) : null;
    }

    @Override
    public boolean containsKey(@CheckForNull Object key) {
      return containsColumn(key);
    }

    @Override
    @CheckForNull
    public Map<R, V> remove(@CheckForNull Object key) {
      return containsColumn(key) ? removeColumn(key) : null;
    }