diff --git a/.travis.yml b/.travis.yml
index 7561b1a..48ae30c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,5 @@
 language: objective-c
+osx_image: xcode7.2
 
 script:
 - make clean test
diff --git a/Makefile b/Makefile
index f941ddd..5cb2622 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,6 @@
-PROJECT = DemoApp/DemoApp.xcodeproj
-SCHEME = DemoApp
-TEST_SDK = iphonesimulator
-CONFIGURATION_DEBUG = Debug
+PROJECT = 'DemoApp/DemoApp.xcodeproj'
+SCHEME = 'DemoApp'
+DESTINATION = 'platform=iOS Simulator,name=iPhone 6s'
 
 clean:
 	xcodebuild \
@@ -12,6 +11,5 @@ test:
 	xcodebuild \
 		-project $(PROJECT) \
 		-scheme $(SCHEME) \
-		-sdk $(TEST_SDK) \
-		-configuration $(CONFIGURATION_DEBUG) \
+		-destination $(DESTINATION) \
 		build test
diff --git a/SBYZipArchive/SBYZipArchive.h b/SBYZipArchive/SBYZipArchive.h
index 9463a96..2df1335 100644
--- a/SBYZipArchive/SBYZipArchive.h
+++ b/SBYZipArchive/SBYZipArchive.h
@@ -11,11 +11,13 @@
 
 @protocol SBYZipArchiveDelegate;
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface SBYZipArchive : NSObject
-@property (strong, nonatomic, readonly) NSURL *url;
-@property (readonly) NSArray *entries;
+@property (nonatomic, readonly) NSURL *url;
+@property (readonly) NSArray<SBYZipEntry *> *entries;
 
-- (id)initWithContentsOfURL:(NSURL *)url error:(NSError *__autoreleasing *)error;
+- (instancetype)initWithContentsOfURL:(NSURL *)url error:(NSError *__autoreleasing *)error;
 
 - (BOOL)loadEntriesWithError:(NSError *__autoreleasing *)error;
 
@@ -23,12 +25,11 @@
 
 - (void)unzipEntry:(SBYZipEntry *)entry
              toURL:(NSURL *)url
-           success:(void (^)(NSURL *unzippedFileLocation))success
-           failure:(void (^)(NSError *error))failure
-          progress:(void (^)(NSUInteger bytesUnzipped, NSUInteger totalBytes))progress;
+           success:(nullable void (^)(NSURL *unzippedFileLocation))success
+           failure:(nullable void (^)(NSError *error))failure
+          progress:(nullable void (^)(NSUInteger bytesUnzipped, NSUInteger totalBytes))progress;
 @end
 
-
 extern NSString* const SBYZipArchiveErrorDomain;
 
 typedef NS_ENUM(NSInteger, SBYZipArchiveError)
@@ -36,4 +37,6 @@ typedef NS_ENUM(NSInteger, SBYZipArchiveError)
     SBYZipArchiveErrorCannotOpenFile = 1,
     SBYZipArchiveErrorCannotGetFileInfo = 2,
     SBYZipArchiveErrorCannotUnzipEntryFile = 3,
-};
\ No newline at end of file
+};
+
+NS_ASSUME_NONNULL_END
diff --git a/SBYZipArchive/SBYZipArchive.m b/SBYZipArchive/SBYZipArchive.m
index 0ddc23a..d7c1693 100644
--- a/SBYZipArchive/SBYZipArchive.m
+++ b/SBYZipArchive/SBYZipArchive.m
@@ -14,17 +14,17 @@
 static const NSUInteger SBYZipArchiveBufferSize = 4096;
 
 @interface SBYZipArchive () <NSStreamDelegate>
-@property (strong, nonatomic, readwrite) NSURL *url;
-@property (assign, nonatomic) unzFile unzFile;
-@property (strong, nonatomic) NSMutableArray *cachedEntries;
+@property (nonatomic, readwrite) NSURL *url;
+@property (nonatomic) unzFile unzFile;
+@property (nonatomic) NSMutableArray<SBYZipEntry *> *cachedEntries;
 
-@property (strong, nonatomic) dispatch_semaphore_t semaphore;
+@property (nonatomic) dispatch_semaphore_t semaphore;
 
-@property (strong, nonatomic) NSOutputStream *outputStream;
-@property (strong, nonatomic) NSURL *unzipDestinationURL;
+@property (nonatomic) NSOutputStream *outputStream;
+@property (nonatomic) NSURL *unzipDestinationURL;
 
-@property (assign, nonatomic) NSUInteger bytesUnzipped;
-@property (assign, nonatomic) NSUInteger totalBytes;
+@property (nonatomic) NSUInteger bytesUnzipped;
+@property (nonatomic) NSUInteger totalBytes;
 
 @property (copy, nonatomic) void (^successBlock)(NSURL *);
 @property (copy, nonatomic) void (^failureBlock)(NSError *);
@@ -33,7 +33,7 @@ @interface SBYZipArchive () <NSStreamDelegate>
 
 @implementation SBYZipArchive
 
-- (id)initWithContentsOfURL:(NSURL *)url error:(NSError *__autoreleasing *)error
+- (instancetype)initWithContentsOfURL:(NSURL *)url error:(NSError *__autoreleasing *)error
 {
     self = [super init];
     if (self) {
@@ -133,7 +133,7 @@ - (BOOL)loadEntriesWithError:(NSError *__autoreleasing *)error
     return YES;
 }
 
-- (void)unzipEntry:(SBYZipEntry *)entry toURL:(NSURL *)url success:(void (^)(NSURL *))success failure:(void (^)(NSError *))failure progress:(void (^)(NSUInteger, NSUInteger))progress
+- (void)unzipEntry:(SBYZipEntry *)entry toURL:(NSURL *)url success:(nullable void (^)(NSURL *))success failure:(nullable void (^)(NSError *))failure progress:(nullable void (^)(NSUInteger, NSUInteger))progress
 {
     if (!entry) {
         return;
diff --git a/SBYZipArchive/SBYZipEntry.h b/SBYZipArchive/SBYZipEntry.h
index 01dc279..8e0af9a 100644
--- a/SBYZipArchive/SBYZipEntry.h
+++ b/SBYZipArchive/SBYZipEntry.h
@@ -10,24 +10,28 @@
 
 @class SBYZipArchive;
 
+NS_ASSUME_NONNULL_BEGIN
+
 @interface SBYZipEntry : NSObject
-@property (weak, nonatomic, readonly)   SBYZipArchive *archive;
-@property (copy, nonatomic, readonly)   NSString  *fileName;
-@property (assign, nonatomic, readonly) NSUInteger fileSize;
-@property (assign, nonatomic, readonly) NSUInteger offset;
+@property (weak, nonatomic, readonly) SBYZipArchive *archive;
+@property (copy, nonatomic, readonly) NSString *fileName;
+@property (nonatomic, readonly) NSUInteger fileSize;
+@property (nonatomic, readonly) NSUInteger offset;
 
-- (id)initWithArchive:(SBYZipArchive *)archive
-             fileName:(NSString *)fileName
-             fileSize:(NSUInteger)fileSize
-               offset:(NSUInteger)offset;
+- (instancetype)initWithArchive:(SBYZipArchive *)archive
+                       fileName:(NSString *)fileName
+                       fileSize:(NSUInteger)fileSize
+                         offset:(NSUInteger)offset;
 
 // To unzip small file synchronously
 - (NSData *)dataWithError:(NSError *__autoreleasing *)error;
 
 // To unzip large file asynchronously
 - (void)unzipToURL:(NSURL *)url
-           success:(void (^)(NSURL *unzippedFileLocation))success
-           failure:(void (^)(NSError *error))failure
-          progress:(void (^)(NSUInteger bytesUnzipped, NSUInteger totalBytes))progress;
+           success:(nullable void (^)(NSURL *unzippedFileLocation))success
+           failure:(nullable void (^)(NSError *error))failure
+          progress:(nullable void (^)(NSUInteger bytesUnzipped, NSUInteger totalBytes))progress;
 
 @end
+
+NS_ASSUME_NONNULL_END
diff --git a/SBYZipArchive/SBYZipEntry.m b/SBYZipArchive/SBYZipEntry.m
index 0258cf7..763a6b8 100644
--- a/SBYZipArchive/SBYZipEntry.m
+++ b/SBYZipArchive/SBYZipEntry.m
@@ -10,25 +10,25 @@
 #import "SBYZipArchive.h"
 
 @interface SBYZipEntry ()
-@property (weak, nonatomic, readwrite)   SBYZipArchive *archive;
-@property (copy, nonatomic, readwrite)   NSString *fileName;
-@property (assign, nonatomic, readwrite) NSUInteger fileSize;
-@property (assign, nonatomic, readwrite) NSUInteger offset;
+@property (weak, nonatomic, readwrite) SBYZipArchive *archive;
+@property (copy, nonatomic, readwrite) NSString *fileName;
+@property (nonatomic, readwrite) NSUInteger fileSize;
+@property (nonatomic, readwrite) NSUInteger offset;
 @end
 
 @implementation SBYZipEntry
 
-- (id)initWithArchive:(SBYZipArchive *)archive
+- (instancetype)initWithArchive:(SBYZipArchive *)archive
              fileName:(NSString *)fileName
              fileSize:(NSUInteger)fileSize
                offset:(NSUInteger)offset
 {
     self = [super init];
     if (self) {
-        self.archive            = archive;
-        self.fileName           = fileName;
-        self.fileSize           = fileSize;
-        self.offset             = offset;
+        self.archive  = archive;
+        self.fileName = fileName;
+        self.fileSize = fileSize;
+        self.offset   = offset;
     }
     return self;
 }
@@ -38,7 +38,7 @@ - (NSData *)dataWithError:(NSError *__autoreleasing *)error
     return [self.archive dataForEntry:self error:error];
 }
 
-- (void)unzipToURL:(NSURL *)url success:(void (^)(NSURL *))success failure:(void (^)(NSError *))failure progress:(void (^)(NSUInteger, NSUInteger))progress
+- (void)unzipToURL:(NSURL *)url success:(nullable void (^)(NSURL *))success failure:(nullable void (^)(NSError *))failure progress:(nullable void (^)(NSUInteger, NSUInteger))progress
 {
     [self.archive unzipEntry:self toURL:url success:success failure:failure progress:progress];
 }