Getting TypeError while implementing the gradient descent code for regularized values

Question

My Code (from coursera):

def gradient_desc(X, Y, w_in, b_in, cost_f, grad_f, alp, num, lambda_):
    m = len(X)
    # An array to store cost J and w's at each iteration primarily for graphing later
    J_history = []
    w_history = []
    for i in range(num):
        # Calculate the gradient and update the parameters
        dj_db, dj_dw = grad_f(X, Y, w_in, b_in, lambda_)
        # Update Parameters using w, b, alpha and gradient
        w_in = w_in - alpha * dj_dw
        b_in = b_in - alpha * dj_db
        # Save cost J at each iteration
        if i<100000:      # prevent resource exhaustion
            J_history.append(cost_f(X, Y, w_in, b_in, lambda_))
        # Print cost every at intervals 10 times or as many iterations if < 10
        if i% math.ceil(num/10) == 0:
            w_history.append(w_in)
            print(f"Iteration {i:4}: Cost {float(J_history[-1]):0.2e}  ")                       

    return w_in, b_in, J_history, w_history #return w and J,w history for graphing

The error:

TypeError: float() argument must be a string or a real number, not 'NoneType'

This is the only bit in the entire code that is bugging (at least bugging till now). How to resolve this?