Adding multiple assets to collection with Photo Framework - Objective-C - ios

Question

I have the following code:
-(void) saveFromLocationsArray: (NSArray*) locations{
[self.photoLib performChanges:^{
PHAssetChangeRequest *creationRequest;
NSMutableArray *changesArray = [[NSMutableArray alloc] init];
for(int i=0; i<locations.count; i++){
creationRequest = [PHAssetChangeRequest creationRequestForAssetFromImageAtFileURL:locations[i]];
[changesArray addObject:[creationRequest placeholderForCreatedAsset]];
}
PHAssetCollectionChangeRequest *collectionChangeRequest = [PHAssetCollectionChangeRequest changeRequestForAssetCollection:self.objcAppAlbum];
[collectionChangeRequest addAssets:changesArray];
} completionHandler:^(BOOL success, NSError *error) {
if(!success){
NSLog(#"an error has occured while trying to save pic to library: %#", error);
}
else{
NSLog(#"image was added to album ObjcApp");
}
}];
In a nutshell: I have an array of locations (URLS) of images I've just downloaded, and I'm trying to add them all to the iPhone's photo library. i managed to do so for one image (in a different method) but here it crashes on [collectionChangeRequest addAssets:changesArray], my guess is that the argument fo addAssest is invalid... but i can't figure out why.
this is the working code for one image only:
-(void) saveFromLocation: (NSURL*) location{
[self.photoLib performChanges:^{
PHAssetChangeRequest *creationRequest = [PHAssetChangeRequest creationRequestForAssetFromImageAtFileURL:location];
PHAssetCollectionChangeRequest *collectionChangeRequest = [PHAssetCollectionChangeRequest changeRequestForAssetCollection:self.objcAppAlbum];
NSArray *arr = [NSArray arrayWithObjects:[creationRequest placeholderForCreatedAsset], nil];
[collectionChangeRequest addAssets:arr];
} completionHandler:^(BOOL success, NSError *error) {
if(!success){
NSLog(#"an error has occured while trying to save pic to library: %#", error);
}
else{
NSLog(#"image was added to album ObjcApp");
}
}];
}
error: 2019-03-05 14:26:26.062368 ObjcProject[1026:371314]
-[__NSCFString path]: unrecognized selector sent to instance 0x1740fa880 2019-03-05 14:26:26.063343 ObjcProject[1026:371314] *
Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: '-[__NSCFString path]:
unrecognized selector sent to instance 0x1740fa880'
* First throw call stack: (0x186d3d1b8 0x18577455c 0x186d44268 0x186d41270 0x186c3a80c 0x19276338c 0x192700ee8 0x19275e8c4
0x19275edfc 0x1914ae4b8 0x191d49c4c 0x19275e510 0x192738be4
0x1914a0ddc 0x1001dd258 0x1001dd218 0x1001eaaec 0x1001e0ce0
0x1001eb088 0x1001ece2c 0x1001ecb78 0x185dcf2a0 0x185dced8c)
libc++abi.dylib: terminating with uncaught exception of type
NSException