diff --git a/GBVersionTracking/GBVersionTracking.h b/GBVersionTracking/GBVersionTracking.h
index 2fe7dbd..d4b0b2f 100644
--- a/GBVersionTracking/GBVersionTracking.h
+++ b/GBVersionTracking/GBVersionTracking.h
@@ -35,61 +35,61 @@ typedef void(^GBVersionTrackingHandlerBlock)(void);
 /**
  Check if this is the first launch, for a particular version number. Useful if you want to execute some code for first time launches of a particular version (like db migrations?).
  */
-+ (BOOL)isFirstLaunchForVersion:(NSString *)version;
++ (BOOL)isFirstLaunchForVersion:(nonnull NSString *)version;
 
 /**
  Check if this is the first launch, for a particular build number. Useful if you want to execute some code for first time launches of a particular version (like db migrations?).
  */
-+ (BOOL)isFirstLaunchForBuild:(NSString *)build;
++ (BOOL)isFirstLaunchForBuild:(nonnull NSString *)build;
 
 /**
  Calls block if the condition is satisfied that the current version matches `version`, and this is the first time this app version is being launched.
  */
-+ (void)callBlockOnFirstLaunchOfVersion:(NSString *)version block:(GBVersionTrackingHandlerBlock)block;
++ (void)callBlockOnFirstLaunchOfVersion:(nonnull NSString *)version block:(nonnull GBVersionTrackingHandlerBlock)block;
 
 /**
  Calls block if the condition is satisfied that the current build matches `build`, and this is the first time this app build is being launched.
  */
-+ (void)callBlockOnFirstLaunchOfBuild:(NSString *)build block:(GBVersionTrackingHandlerBlock)block;
++ (void)callBlockOnFirstLaunchOfBuild:(nonnull NSString *)build block:(nonnull GBVersionTrackingHandlerBlock)block;
 
 /**
  Returns the current version of the app, as defined in the PList, e.g. "4.3".
  */
-+ (NSString *)currentVersion;
++ (nonnull NSString *)currentVersion;
 
 /**
  Returns the previous version of the app, as defined in the PList, e.g. "4.3".
  */
-+ (NSString *)previousVersion;
++ (nullable NSString *)previousVersion;
 
 /**
  Returns the version which the user first installed the app at.
  */
-+ (NSString *)firstInstalledVersion;
++ (nullable NSString *)firstInstalledVersion;
 
 /**
  Returns a sorted array of versions which the user has had installed, e.g. ["3.5", "4.0", "4.1"]. In terms of ordering, more recent versions are appended at the back of the array. The array includes the current version as the last element.
  */
-+ (NSArray *)versionHistory;
++ (nullable NSArray *)versionHistory;
 
 /**
  Returns the current build of the app, as defined in the PList, e.g. "4300".
  */
-+ (NSString *)currentBuild;
++ (nonnull NSString *)currentBuild;
 
 /**
  Returns the previous build of the app, as defined in the PList, e.g. "4300".
  */
-+ (NSString *)previousBuild;
++ (nullable NSString *)previousBuild;
 
 /**
  Returns the version which the user first installed the app at.
  */
-+ (NSString *)firstInstalledBuild;
++ (nullable NSString *)firstInstalledBuild;
 
 /**
  Returns a sorted array of builds which the user has had installed, e.g. ["3500", "4000", "4100"]. In terms of ordering, more recent builds are appended at the back of the array.  The array includes the current build as the last element.
  */
-+ (NSArray *)buildHistory;
++ (nullable NSArray *)buildHistory;
 
 @end