Interactive Brokers, python code for combo trades via the IB API

combo_spreads_file.py

#Reference https://interactivebrokers.github.io/tws-api/basic_orders.html#limitordercombolegprices
def build_combo_contract(info):
    contract = Contract()
    contract.symbol = 'ACN'
    contract.secType = "BAG"
    contract.currency = "USD"
    contract.exchange = "BOX"
  
    leg1 = ComboLeg()
    leg1.conId = info['contractId_0']
    leg1.ratio = 1
    leg1.action = "BUY"
    leg1.exchange = "BOX"

    leg2 = ComboLeg()
    leg2.conId = info['contractId_1']
    leg2.ratio = 1
    leg2.action = "SELL"
    leg2.exchange = "BOX"

    contract.comboLegs = []
    contract.comboLegs.append(leg1)
    contract.comboLegs.append(leg2)

    return contract

def make_order_block(data):
    quantity = 1
    order = Order()
    order.action = "BUY"
    order.orderType = "LMT"
    order.orderComboLegs = []
    order.totalQuantity = quantity
    legPrices = [18.90,15.50]
    for price in legPrices:
        comboLeg = OrderComboLeg()
        comboLeg.price = price
        order.orderComboLegs.append(comboLeg)

    order.smartComboRoutingParams = []
    order.smartComboRoutingParams.append(TagValue("NonGuaranteed", "1"))


comboContract=order_contract_library.build_combo_contract(combo)
blockCombo =order_contract_library.make_order_block(combo)
orderId=123
self.placeOrder(orderId, comboContract,blockCombo)

Error code: 
Error. Id: 121316 Code: 463 Msg: You must enter a valid price.