expr not of form a*x**b

Raise code

    from sympy import powsimp
    (c, m) = expand_power_base(powsimp(expr)).as_coeff_mul(x)
    if not m:
        return c, S.Zero
    [m] = m
    if m.is_Pow:
        if m.base != x:
            raise _CoeffExpValueError('expr not of form a*x**b')
        return c, m.exp
    elif m == x:
        return c, S.One
    else:
        raise _CoeffExpValueError('expr not of form a*x**b: %s' % expr)