diff --git a/iOS/MMKV/MMKV/MMKV.h b/iOS/MMKV/MMKV/MMKV.h
index 68f86bd0..2830a734 100644
--- a/iOS/MMKV/MMKV/MMKV.h
+++ b/iOS/MMKV/MMKV/MMKV.h
@@ -37,7 +37,7 @@ NS_ASSUME_NONNULL_BEGIN
 + (nullable instancetype)mmkvWithID:(NSString *)mmapID cryptKey:(nullable NSData *)cryptKey NS_SWIFT_NAME(init(mmapID:cryptKey:));
 
 - (BOOL)reKey:(nullable NSData *)newKey NS_SWIFT_NAME(reset(cryptKey:));
-- (NSData *)cryptKey;
+- (nullable NSData *)cryptKey;
 
 // object: NSString/NSData/NSDate
 - (BOOL)setObject:(id)object forKey:(NSString *)key NS_SWIFT_NAME(set(_:forKey:));
@@ -89,7 +89,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)removeValueForKey:(NSString *)key NS_SWIFT_NAME(removeValue(forKey:));
 
-- (void)removeValuesForKeys:(NSArray *)arrKeys NS_SWIFT_NAME(removeValues(forKeys:));
+- (void)removeValuesForKeys:(NSArray<NSString *> *)arrKeys NS_SWIFT_NAME(removeValues(forKeys:));
 
 - (void)clearMemoryCache;
 
diff --git a/iOS/MMKV/MMKV/MMKV.mm b/iOS/MMKV/MMKV/MMKV.mm
index 16bf8f5b..769fd11c 100644
--- a/iOS/MMKV/MMKV/MMKV.mm
+++ b/iOS/MMKV/MMKV/MMKV.mm
@@ -754,7 +754,7 @@ - (void)prepareCRCFile {
 
 #pragma mark - encryption & decryption
 
-- (NSData *)cryptKey {
+- (nullable NSData *)cryptKey {
 	if (m_cryptor) {
 		NSMutableData *data = [NSMutableData dataWithLength:AES_KEY_LEN];
 		m_cryptor->getKey(data.mutableBytes);