docplex问题：TypeError：无法使用==来测试表达式是否相等，请尝试使用Python等于运算符或方法等于：

mymodel = Model("OPL_Purchasing_plan");
nr_month= range(0,12)
na_pur_req = [550,750,6500,675,210,120,560,140,320,100]
na_average_cost = 12.5,12.5,12.5]

for i in nr_month:
    if(na_pur_req[i] == 0):
        mymodel.add_constraint(na_actual_cost_test[0][i] <= na_actual_suply[0][i] * 0,'con9');
     
    else:
        mymodel.add_constraint(na_actual_cost_test[0][i] <= na_actual_suply[0][i] * na_average_cost[i],'con10');

我收到如下错误

TypeError: Cannot use == to test expression equality,try using Python is operator or method equals

为什么我会收到此错误。