Black-Scholes implied vol BelowIntrinsicError with negative risk-free rate

[stnatter]

Trying to calculate the implied vol for Eurostoxx50 options and running into problems due to the negative risk-free rate in EUR:

from py_vollib.black_scholes.implied_volatility import implied_volatility
implied_volatility(price=107.35, S=3576.1, K=3575, t=0.139726, r=-0.618873, flag='p')

Any suggestions how to handle/overcome this?

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BelowIntrinsicException Traceback (most recent call last)
in
1 from py_vollib.black_scholes.implied_volatility import implied_volatility
----> 2 implied_volatility(price=107.35, S=3576.1, K=3575, t=0.139726, r=-0.618873, flag='p')

~/miniconda3/lib/python3.8/site-packages/py_vollib/black_scholes/implied_volatility.py in implied_volatility(price, S, K, t, r, flag)
89 undiscounted_option_price = price / deflater
90 F = forward_price(S, t, r)
---> 91 sigma_calc = iv(undiscounted_option_price, F, K, t, binary_flag[flag])
92 if sigma_calc == FLOAT_MAX:
93 raise PriceIsAboveMaximum()

~/miniconda3/lib/python3.8/site-packages/py_lets_be_rational/lets_be_rational.py in implied_volatility_from_a_transformed_rational_guess(price, F, K, T, q)
650 :rtype: float
651 """
--> 652 return implied_volatility_from_a_transformed_rational_guess_with_limited_iterations(
653 price, F, K, T, q, implied_volatility_maximum_iterations)
654

~/miniconda3/lib/python3.8/site-packages/py_lets_be_rational/lets_be_rational.py in implied_volatility_from_a_transformed_rational_guess_with_limited_iterations(price, F, K, T, q, N)
601 intrinsic = fabs(max(K - F if q < 0 else F - K, 0.0))
602 if price < intrinsic:
--> 603 raise BelowIntrinsicException
604 max_price = K if q < 0 else F
605 if price >= max_price:

BelowIntrinsicException: The volatility is below the intrinsic value.

[stnatter]

mea culpa. breaks of course down with non-sensical rates