specify dummy variables for %s. If the function contains more than one free symbol, a dummy variable should be supplied explicitly e.g. FourierSeries(m*n**2, (n, -pi, pi))

Raise code

d_x(func):
        free = func.free_symbols
        if len(free) == 1:
            return free.pop()
        elif not free:
            return Dummy('k')
        else:
            raise ValueError(
                " specify dummy variables for %s. If the function contains"
                " more than one free symbol, a dummy variable should be"
                " supplied explicitly e.g. FourierSeries(m*n**2, (n, -pi, pi))"
                % func)

    x, start, stop = None, None, None
    if limits is None:
        x, s