"Invalid initializer" error when compiling

I use this code

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    

    // Override point for customization after application lau开发者_StackOverflow中文版nch.

    [self.window makeKeyAndVisible];

    // position is a number between 0 and 1, where 0 gives the start position, and 1 gives the end position

    CLLocationCoordinate2D startPoint = CLLocationCoordinate2DMake(37.9,-122.5);
    CLLocationCoordinate2D endPoint = CLLocationCoordinate2DMake(37.188022,-122.39979);

    for (int i = 1; i < 5; i++) {
        float position = i / 5.0;
    CLLocationCoordinate2D middlePosition =  [self pointBetweenStartPoint:startPoint endPoint:endPoint position:position];
    NSLog("%f, %f", middlePosition.latitude, middlePosition.longitude);
    }

    return YES;
}

- (CLLocationCoordinate2D)pointBetweenStartPoint:(CLLocationCoordinate2D)startPoint endPoint:(CLLocationCoordinate2D)endPoint position:(float)position {

    CLLocationDegrees latSpan = endPoint.latitude - startPoint.latitude;
    CLLocationDegrees longSpan = endPoint.longitude - startPoint.longitude;

    CLLocationCoordinate2D ret = CLLocationCoordinate2DMake(startPoint.latitude + latSpan*position,
startPoint.longitude + longSpan*position);
    return ret;
}

and getting this error: invalid initializer in this line

CLLocationCoordinate2D middlePosition = [self pointBetweenStartPoint:startPoint endPoint:endPoint position:position];

How remove it?

Is there a forward declaration of pointBetweenStartPoint:endPoint:position: visible (e.g. from an imported header file) before the block of code you quoted? If not, the compiler will assume the method returns id, which is not valid for initializing a CLLocationCoordinate2D struct, and give that error.