invalid literal for int() value?

TICKET_PRICE = 10
SERVICE_CHARGE = 2
tickets_remaining = 100  

# Create the calculate_price function. It takes number of tickets and returns num_tickets * TICKET_PRICE
def calculate_price(number_of_tickets):
    #creare a new constant for 2 dollar service charge
    # Add the service charge to the result
    return (number_of_tickets * TICKET_PRICE) + SERVICE_CHARGE

while tickets_remaining >= 1:    
    print("There are {} tickets remaining.".format(tickets_remaining))    
    name = input("What is your name:  ")
    num_tickets = input("Hello {}, How many tickets do you need?:  ".format(name))
    try:        
        num_tickets = int(num_tickets)
        if num_tickets > tickets_remaining:
            raise ValueError("There are only {} tickets remaining".format(tickets_remaining))
    except ValueError as err:
        print("Oh no. We ran into an issue. {}. Please try again.".format(err))
    else:    
        ticket_price = calculate_price(num_tickets)
        print("For {} tickets, the total price is {}".format(num_tickets, ticket_price))
        prompt_answer = input("Do you want to purchase the ticket(s)? Y/N    ")
        if prompt_answer.lower() == "y":
            print("Purchase of {} tickets confirmed. SOLD!".format(num_tickets))
            tickets_remaining -= num_tickets  
        else:
            print("Thanks for showing interest in us, {}".format(name))
print("Sorry the tickets are all sold out! :(")