diff --git a/CTAssetsPickerController/CTAssetCollectionViewCell.h b/CTAssetsPickerController/CTAssetCollectionViewCell.h
index 9857b49b..945a0ff8 100644
--- a/CTAssetsPickerController/CTAssetCollectionViewCell.h
+++ b/CTAssetsPickerController/CTAssetCollectionViewCell.h
@@ -28,7 +28,7 @@
 #import <Photos/Photos.h>
 #import "CTAssetThumbnailStacks.h"
 
-
+NS_ASSUME_NONNULL_BEGIN
 
 @interface CTAssetCollectionViewCell : UITableViewCell
 
@@ -51,4 +51,6 @@
 - (instancetype)initWithThumbnailSize:(CGSize)size reuseIdentifier:(nullable NSString *)reuseIdentifier;
 - (void)bind:(nonnull PHAssetCollection *)collection count:(NSUInteger)count;
 
-@end
\ No newline at end of file
+@end
+
+NS_ASSUME_NONNULL_END
\ No newline at end of file
diff --git a/CTAssetsPickerController/CTAssetThumbnailStacks.h b/CTAssetsPickerController/CTAssetThumbnailStacks.h
index e2280691..6a5ff4c3 100644
--- a/CTAssetsPickerController/CTAssetThumbnailStacks.h
+++ b/CTAssetsPickerController/CTAssetThumbnailStacks.h
@@ -27,6 +27,8 @@
 #import <UIKit/UIKit.h>
 #import "CTAssetThumbnailView.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface CTAssetThumbnailStacks : UIView
 
 @property (nonatomic, assign) CGSize thumbnailSize;
@@ -37,3 +39,5 @@
 - (void)setHighlighted:(BOOL)highlighted;
 
 @end
+
+NS_ASSUME_NONNULL_END
\ No newline at end of file
diff --git a/CTAssetsPickerController/CTAssetsGridViewController.h b/CTAssetsPickerController/CTAssetsGridViewController.h
index 34d68fec..fc1f45d0 100644
--- a/CTAssetsPickerController/CTAssetsGridViewController.h
+++ b/CTAssetsPickerController/CTAssetsGridViewController.h
@@ -42,7 +42,7 @@
 
 @interface CTAssetsGridViewController : UICollectionViewController
 
-@property (nonatomic, weak) id<CTAssetsGridViewControllerDelegate> delegate;
+@property (nonatomic, weak, nullable) id<CTAssetsGridViewControllerDelegate> delegate;
 @property (nonatomic, strong, nonnull) PHAssetCollection *assetCollection;
 
 @end