关于objective-c中NSNumber的问题

爆出错误：
Incompatible pointer types assigning to 'UITextfield _strong' from 'NSNumber _strong

代码：

.h文件
@property (weak, nonatomic) IBOutlet UITextField *initialBudget;
@property (weak, nonatomic) IBOutlet UITextField *expenses;
@property (weak, nonatomic) IBOutlet UITextField *timeSpent;
@property (weak, nonatomic) IBOutlet UITextField *incomePerHour

计算：

- (IBAction)calculateResults:(id)sender {
    double budget = [initialBudget.text doubleValue ];
    double expense = [expenses.text doubleValue];
    double time = [timeSpent.text doubleValue];
    double hourlyIncome = (budget - expense)/time;
    NSNumber *resultNumber = [[NSNumber alloc] initWithDouble:hourlyIncome];
    incomePerHour = resultNumber;
};