call to super method causes precondition violation

let's consider this valid piece of precondition overriding:

class Account 
{
    protected BigDecimal balance

    def Account( BigDecimal amount = 0.0 )
    {
        balance = amount
    }

    void deposit( BigDecimal amount )
    {
        balance += amount
    }

    @Requires({ amount >= 0.0 })
    @Ensures({ balance == old.balance - amount })
    BigDecimal withdraw( BigDecimal amount )
    {
        if (balance < amount) return 0.0
        
        balance -= amount
        return amount
    }

    BigDecimal getBalance()
    {
        return balance
    }
}  

class BetterAccount extends Account {
    
    @Requires({ true })
    BigDecimal withdraw( BigDecimal amount )
    {
        return super.withdraw (amount)
    }
}

in the case above, betterAccount.withdraw (null) fails (it shouldn't) because a call to super causes a fresh revalidation