diff --git a/Blog/NotificationToSelectorMap.h b/Blog/NotificationToSelectorMap.h
index b286516..d43744f 100644
--- a/Blog/NotificationToSelectorMap.h
+++ b/Blog/NotificationToSelectorMap.h
@@ -10,10 +10,10 @@
 
 @interface NotificationToSelectorMap : NSObject
 
-@property (nonatomic, copy) NSDictionary *notificationNameToSelectorNameMap;
+@property (nonatomic, copy, nonnull) NSDictionary *notificationNameToSelectorNameMap;
 
-- (instancetype)initWithNotificationMap:(nonnull NSDictionary *)notificationMap;
-- (void)addObserver:(NSObject *)observer;
-- (void)removeObserver:(NSObject *)observer;
+- (nonnull instancetype)initWithNotificationMap:(nonnull NSDictionary *)notificationMap;
+- (void)addObserver:(nonnull NSObject *)observer;
+- (void)removeObserver:(nonnull NSObject *)observer;
 
 @end
diff --git a/Blog/NotificationToSelectorMap.m b/Blog/NotificationToSelectorMap.m
index bfec403..4f64b7b 100644
--- a/Blog/NotificationToSelectorMap.m
+++ b/Blog/NotificationToSelectorMap.m
@@ -10,7 +10,7 @@
 
 @implementation NotificationToSelectorMap
 
-- (instancetype)initWithNotificationMap:(nonnull NSDictionary *)notificationMap {
+- (nonnull instancetype)initWithNotificationMap:(nonnull NSDictionary *)notificationMap {
     self = [super init];
     if (self) {
         _notificationNameToSelectorNameMap = notificationMap;
@@ -18,7 +18,7 @@
     return self;
 }
 
-- (void)addObserver:(NSObject *)observer {
+- (void)addObserver:(nonnull NSObject *)observer {
     NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
     for (NSString *notificationName in self.notificationNameToSelectorNameMap.allKeys) {
         NSString *selectorName = self.notificationNameToSelectorNameMap[notificationName];
@@ -26,7 +26,7 @@
     }
 }
 
-- (void)removeObserver:(NSObject *)observer {
+- (void)removeObserver:(nonnull NSObject *)observer {
     NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
     for (NSString *notificationName in self.notificationNameToSelectorNameMap.allKeys) {
         [notificationCenter removeObserver:observer name:notificationName object:nil];